C/C++ :: Isdigit To Work With Negative Numbers

Ok well i know that isdigit in my code is seeing the negatives as a character and so i made my program give an error message. But i need negative numbers to work how could i get this to work with negative numbers.

The code is supposed to add the 3 numbers input into the command line argument including negative numbers.

#include <stdio.h>
#include <stdlib.h> 
#include <ctype.h>
#include <string.h>    
int main(int argc, char *argv[]){  
   int add=0;
   int i;
 
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