diff --git a/arith/Makefile b/arith/Makefile
new file mode 100644
index 0000000..a183b5b
--- /dev/null
+++ b/arith/Makefile
@@ -0,0 +1,11 @@
+TARGETS = adder
+
+.PHONY: all clean
+
+all: $(TARGETS)
+
+clean:
+	rm -rf $(TARGETS)
+
+adder:
+	gcc -I../common/ ../common/auxiliar.c adder.c -o adder 
\ No newline at end of file
diff --git a/arith/adder.c b/arith/adder.c
new file mode 100644
index 0000000..c890fa1
--- /dev/null
+++ b/arith/adder.c
@@ -0,0 +1,53 @@
+#include <stdio.h>
+#include <string.h>
+#include "auxiliar.h"
+
+int check_length(char **inputs, uint8_t num_inputs)
+{
+	int input_length = strlen(inputs[0]);
+    if((input_length != strlen(inputs[1])) || (strlen(inputs[2]) != 1))
+    {
+    	return -1;
+    }
+    return input_length;
+}
+
+int main(int argc, char **argv)
+{
+	int i;
+	int pos;
+	if(argc != 2)
+	{
+		error();
+	}
+	char *input = argv[1];
+	if(!strcmp(input,"error"))
+	{
+		error();
+	}
+	char **inputs = comma_separate(input);
+	if(inputs == NULL)
+	{
+		error();
+	}
+	int num_inputs = num_occurrences(input,',');
+	if(num_inputs != 3)
+	{
+		error();
+	}
+	int input_length = check_length(inputs,num_inputs);
+	if(input_length == -1)
+	{
+		error();
+	}
+	int a,b,carry_in,c;
+	a = bin2dec(inputs[0]);
+	b = bin2dec(inputs[1]);
+	carry_in = bin2dec(inputs[2]);
+	c = a+b+carry_in;
+	char *result = dec2bin(c,input_length+1);
+	printf("%s,%c",result+1,result[0]);
+	free(result);
+	free_mem(inputs,num_inputs);
+	return 0;
+}
diff --git a/common/auxiliar.c b/common/auxiliar.c
index 8230ad4..4a1200a 100644
--- a/common/auxiliar.c
+++ b/common/auxiliar.c
@@ -4,7 +4,7 @@ char* substring(char *string, uint8_t start, uint8_t end)
 {
         uint8_t size = end - start;
         char *substr;
-        substr = (char*) malloc(sizeof(char)*size + 1);
+        substr = (char*) malloc(sizeof(char)*(size + 1));
         memset(substr,0x00,size+1);
         strncpy(substr,string+start,size);
         return substr;
@@ -60,7 +60,7 @@ char** comma_separate(char *input)
                 return NULL;
         }
         char **inputs;
-        inputs = malloc(sizeof(char*) * input_num);
+        inputs = malloc(sizeof(char*) * (input_num+1));
         for(i = 0; i < input_num; i++)
         {
                 next_pos += next_occurrence(input+pos,',');
@@ -128,14 +128,14 @@ int bin2dec(char *bin)
         int num = 0;
         int length = strlen(bin);
         int pos = length - 1;
-        char *aux = (char*) malloc(sizeof(char) * length);
+        char *aux = (char*) malloc(sizeof(char) * (length+1));
         strcpy(aux,bin);
         normalize(&aux,1);
         for(i = 0; i < length; i++)
         {
                 if(aux[pos--])
                 {
-                        num += pow(2,i);
+                        num += 0x1 << i;
                 }
         }
         free(aux);
@@ -144,7 +144,7 @@ int bin2dec(char *bin)
 
 char* dec2bin(int dec, int length)
 {
-        char *bin = (char*) malloc(sizeof(char) * length + 1);
+        char *bin = (char*) malloc(sizeof(char) * (length + 1));
         int pos = length-1;
         int i;
         for(i = 0; i < length; i++)
@@ -160,4 +160,28 @@ char* dec2bin(int dec, int length)
                 dec >>= 1;
         }
         return bin;
-}
\ No newline at end of file
+}
+
+
+void expand_input(char **input, int required_length)
+{
+        char *new_input = malloc(sizeof(char)*(required_length + 1));
+        memset(new_input,'0',required_length+1);
+        int offset = required_length - strlen(*input);
+        strcpy(new_input + offset,*input);
+        free(*input);
+        *input = new_input;
+}
+
+int str2int(char *string)
+{
+        int i;
+        int num = 0;
+        int mul = 1;
+        for(i = 0; i < strlen(string); i++)
+        {
+                num += mul*(string[i] - 48);
+                mul *= 10;
+        }
+        return num;
+}
diff --git a/common/auxiliar.h b/common/auxiliar.h
index 1633f57..e98d974 100644
--- a/common/auxiliar.h
+++ b/common/auxiliar.h
@@ -8,10 +8,12 @@ int next_occurrence(char *string, char character);
 int num_occurrences(char *string, char character);
 int last_occurrence(char *string, char character);
 char** comma_separate(char *input);
+int str2int(char *string);
 
 void error();
 void free_mem(char **inputs, int num_inputs);
 
 void normalize(char **inputs, int num_inputs);
 int bin2dec(char *bin);
-char* dec2bin(int dec, int length);
\ No newline at end of file
+char* dec2bin(int dec, int length);
+void expand_input(char **input, int required_length);
diff --git a/logic_gate/logic_gate.c b/logic_gate/logic_gate.c
index e9ef26b..d82fae4 100644
--- a/logic_gate/logic_gate.c
+++ b/logic_gate/logic_gate.c
@@ -21,8 +21,8 @@ char* or(char **inputs, int num_inputs, int input_length)
 {
 	int i;
 	int j;
-	char *result = malloc(sizeof(char) * input_length + 1);
-	memset(result, 0x00, input_length);
+	char *result = malloc(sizeof(char) * (input_length + 1));
+	memset(result, 0x00, input_length+1);
 	for(i = 0; i < input_length; i++)
 	{
 		for(j = 0; j < num_inputs; j++)
@@ -46,8 +46,8 @@ char* nor(char **inputs, int num_inputs, int input_length)
 {
 	int i;
 	int j;
-	char *result = malloc(sizeof(char) * input_length + 1);
-	memset(result, 0x00, input_length);
+	char *result = malloc(sizeof(char) * (input_length + 1));
+	memset(result, 0x00, input_length+1);
 	for(i = 0; i < input_length; i++)
 	{
 		for(j = 0; j < num_inputs; j++)
@@ -71,8 +71,8 @@ char* xor(char **inputs, int num_inputs, int input_length)
 {
 	int i;
 	int j;
-	char *result = malloc(sizeof(char) * input_length + 1);
-	memset(result, 0x00, input_length);
+	char *result = malloc(sizeof(char) * (input_length + 1));
+	memset(result, 0x00, input_length+1);
 	for(i = 0; i < input_length; i++)
 	{
 		for(j = 0; j < num_inputs; j++)
@@ -95,8 +95,8 @@ char* xnor(char **inputs, int num_inputs, int input_length)
 {
 	int i;
 	int j;
-	char *result = malloc(sizeof(char) * input_length + 1);
-	memset(result, 0x00, input_length);
+	char *result = malloc(sizeof(char) * (input_length + 1));
+	memset(result, 0x00, input_length+1);
 	for(i = 0; i < input_length; i++)
 	{
 		for(j = 0; j < num_inputs; j++)
@@ -119,8 +119,8 @@ char* and(char **inputs, int num_inputs, int input_length)
 {
 	int i;
 	int j;
-	char *result = malloc(sizeof(char) * input_length + 1);
-	memset(result, 0x01, input_length);
+	char *result = malloc(sizeof(char) * (input_length + 1));
+	memset(result, 0x01, input_length+1);
 	for(i = 0; i < input_length; i++)
 	{
 		for(j = 0; j < num_inputs; j++)
@@ -143,8 +143,8 @@ char* nand(char **inputs, int num_inputs, int input_length)
 {
 	int i;
 	int j;
-	char *result = malloc(sizeof(char) * input_length + 1);
-	memset(result, 0x01, input_length);
+	char *result = malloc(sizeof(char) * (input_length + 1));
+	memset(result, 0x01, input_length+1);
 	for(i = 0; i < input_length; i++)
 	{
 		for(j = 0; j < num_inputs; j++)
@@ -167,8 +167,8 @@ char* not(char **inputs, int num_inputs, int input_length)
 {
 	int i;
 	int j;
-	char *result = malloc(sizeof(char) * input_length + 1);
-	memset(result, 0x01, input_length);
+	char *result = malloc(sizeof(char) * (input_length + 1));
+	memset(result, 0x01, input_length+1);
 	for(i = 0; i < input_length; i++)
 	{
 		for(j = 0; j < num_inputs; j++)
diff --git a/plexers/Makefile b/plexers/Makefile
index 69be5b7..3de9494 100644
--- a/plexers/Makefile
+++ b/plexers/Makefile
@@ -1,4 +1,4 @@
-TARGETS = multiplexor demultiplexor encoder decoder
+TARGETS = multiplexor demultiplexor encoder decoder bit_selector
 
 .PHONY: all clean
 
@@ -8,12 +8,14 @@ clean:
 	rm -rf $(TARGETS)
 
 multiplexor:
-	gcc -I../common/ ../common/auxiliar.c multiplexor.c -o multiplexor -lm
+	gcc -I../common/ ../common/auxiliar.c multiplexor.c -o multiplexor 
 
 demultiplexor:
-		gcc -I../common/ ../common/auxiliar.c demultiplexor.c -o demultiplexor -lm
+		gcc -I../common/ ../common/auxiliar.c demultiplexor.c -o demultiplexor 
 
 encoder:
-		gcc -I../common/ ../common/auxiliar.c encoder.c -o encoder -lm
+		gcc -I../common/ ../common/auxiliar.c encoder.c -o encoder 
 decoder:
-		gcc -I../common/ ../common/auxiliar.c decoder.c -o decoder -lm
+		gcc -I../common/ ../common/auxiliar.c decoder.c -o decoder 
+bit_selector:
+		gcc -I../common/ ../common/auxiliar.c bit_selector.c -o bit_selector 
diff --git a/plexers/bit_selector.c b/plexers/bit_selector.c
new file mode 100644
index 0000000..44b8285
--- /dev/null
+++ b/plexers/bit_selector.c
@@ -0,0 +1,72 @@
+#include <stdio.h>
+#include <string.h>
+#include "auxiliar.h"
+#include <math.h>
+
+int check_length(char **inputs, int num_inputs, int output_length)
+{
+	int input_length = strlen(inputs[0]);
+	int selector_length = strlen(inputs[1]);
+	int sel_combinations = 0x1 << selector_length;
+	int num_groups = ceil(((float)input_length)/output_length);
+	int required_combinations = 1;
+    int i = 0;
+    while(required_combinations < num_groups)
+    {
+    	required_combinations = 0x1 << ++i;
+    }
+	if(sel_combinations < required_combinations)
+	{
+		return -1;
+	}
+	return required_combinations * output_length;
+}
+
+int main(int argc, char **argv)
+{
+	int i;
+	int j;
+	if(argc != 3)
+	{
+		error();
+	}
+	char *input = argv[1];
+	int output_length = str2int(argv[2]);
+	if(!strcmp(input,"error"))
+	{
+		error();
+	}
+	char **inputs = comma_separate(input);
+	if(inputs == NULL)
+	{
+		error();
+	}
+	int num_inputs = num_occurrences(input,',');
+	if(num_inputs != 2)
+	{
+		error();
+	}
+	int input_length = strlen(inputs[0]);
+	int required_length = check_length(inputs,num_inputs,output_length);
+	if(required_length == -1)
+	{
+		error();
+	}
+	if(input_length != required_length)
+	{
+		expand_input(&inputs[0],required_length);
+	}
+	char *result = malloc(sizeof(char) * (output_length + 1));
+	memset(result,0x00,output_length+1);
+	int selected = bin2dec(inputs[1]);
+	int pos = 0;
+	for(i = (selected+1)*(output_length)-1; i >= selected*output_length; i--)
+	{
+		result[pos++] = inputs[0][required_length-(i+1)]; 
+	}
+	printf("%s",result);
+	free(result);
+	free_mem(inputs,num_inputs);
+	return 0;
+}
+
diff --git a/plexers/decoder.c b/plexers/decoder.c
index 0b193cd..d4ed33e 100644
--- a/plexers/decoder.c
+++ b/plexers/decoder.c
@@ -26,7 +26,7 @@ int main(int argc, char **argv)
 		error();
 	}
 	int selected = bin2dec(inputs[0]);
-	int num_outputs = pow(2,strlen(inputs[0]));
+	int num_outputs = 0x1 << strlen(inputs[0]);
 	for(i = 0; i < num_outputs; i++)
 	{
 		if(i == selected)
diff --git a/plexers/demultiplexor.c b/plexers/demultiplexor.c
index 78547d1..ef52c2b 100644
--- a/plexers/demultiplexor.c
+++ b/plexers/demultiplexor.c
@@ -28,7 +28,7 @@ int main(int argc, char **argv)
 	}
 	int input_length = strlen(inputs[0]);
 	int selected = bin2dec(inputs[1]);
-	int num_outputs = pow(2,strlen(inputs[1]));
+	int num_outputs = 0x1 << strlen(inputs[1]);
 	for(i = 0; i < num_outputs; i++)
 	{
 		if(i == selected)
diff --git a/plexers/encoder.c b/plexers/encoder.c
index c0086f2..82bbf11 100644
--- a/plexers/encoder.c
+++ b/plexers/encoder.c
@@ -17,7 +17,7 @@ int check_length(char **inputs, uint8_t num_inputs)
     i = 0;
     while(max_value < num_inputs)
     {
-    	max_value = pow(2,++i);
+    	max_value = 0x1 << ++i;
     }
     return i;
 }
diff --git a/plexers/multiplexor.c b/plexers/multiplexor.c
index 1207657..f04d8dd 100644
--- a/plexers/multiplexor.c
+++ b/plexers/multiplexor.c
@@ -15,7 +15,7 @@ int check_length(char **inputs, uint8_t num_inputs)
                 }
         }
         int sel_length = strlen(inputs[num_inputs-1]);
-        if(pow(2,sel_length) < (num_inputs-1))
+        if((0x1 << sel_length) < (num_inputs-1))
         {
         	return -1;
         }
diff --git a/todo b/todo
index 3088924..c593e23 100644
--- a/todo
+++ b/todo
@@ -1,5 +1,3 @@
-bit selector
-
 adder
 substractor
 multiplier