diff --git a/arith/Makefile b/arith/Makefile
index a183b5b..b752a6e 100644
--- a/arith/Makefile
+++ b/arith/Makefile
@@ -1,4 +1,4 @@
-TARGETS = adder
+TARGETS = adder substractor
 
 .PHONY: all clean
 
@@ -8,4 +8,6 @@ clean:
 	rm -rf $(TARGETS)
 
 adder:
-	gcc -I../common/ ../common/auxiliar.c adder.c -o adder 
\ No newline at end of file
+	gcc -I../common/ ../common/auxiliar.c adder.c -o adder 
+substractor:
+	gcc -I../common/ ../common/auxiliar.c substractor.c -o substractor 
\ No newline at end of file
diff --git a/arith/substractor.c b/arith/substractor.c
new file mode 100644
index 0000000..f3071a2
--- /dev/null
+++ b/arith/substractor.c
@@ -0,0 +1,60 @@
+#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,borrow_in,c;
+	char borrow_out = '0';
+	a = bin2dec(inputs[0]);
+	b = bin2dec(inputs[1]);
+	borrow_in = bin2dec(inputs[2]);
+	c = a-(b+borrow_in);
+	if(c < 0)
+	{
+		borrow_out = '1';
+	}
+	char *result = dec2bin(c,input_length);
+	printf("%s,%c",result,borrow_out);
+	free(result);
+	free_mem(inputs,num_inputs);
+	return 0;*/
+	printf("%d",bin2dec("11000000"));
+	return 0;
+}
diff --git a/common/auxiliar.c b/common/auxiliar.c
index 4a1200a..b6cb383 100644
--- a/common/auxiliar.c
+++ b/common/auxiliar.c
@@ -126,10 +126,16 @@ int bin2dec(char *bin)
 {
         int i;
         int num = 0;
+        int factor = 1;
         int length = strlen(bin);
         int pos = length - 1;
         char *aux = (char*) malloc(sizeof(char) * (length+1));
         strcpy(aux,bin);
+        if(aux[0] == '1')
+        {       
+                reverse_two_complement(aux);
+                factor = -1;
+        }
         normalize(&aux,1);
         for(i = 0; i < length; i++)
         {
@@ -138,6 +144,7 @@ int bin2dec(char *bin)
                         num += 0x1 << i;
                 }
         }
+        num *= factor;
         free(aux);
         return num;
 }
@@ -185,3 +192,22 @@ int str2int(char *string)
         }
         return num;
 }
+
+void reverse_two_complement(char *bin)
+{
+        int i;
+        for(i = 0; i < strlen(bin); i++)
+        {
+                if(bin[i] == '0')
+                {
+                        bin[i] = '1';
+                }
+                else
+                {
+                        bin[i] = '0';
+                }
+        }
+        char *bin_added = dec2bin(bin2dec(bin)+1,strlen(bin));
+        strcpy(bin,bin_added);
+        free(bin_added);
+}
\ No newline at end of file
diff --git a/common/auxiliar.h b/common/auxiliar.h
index e98d974..e5dc34e 100644
--- a/common/auxiliar.h
+++ b/common/auxiliar.h
@@ -17,3 +17,4 @@ void normalize(char **inputs, int num_inputs);
 int bin2dec(char *bin);
 char* dec2bin(int dec, int length);
 void expand_input(char **input, int required_length);
+void reverse_two_complement(char *bin);