help fixing my calculator in C

[kiki021600]

This is the code I wrote for a calculator in C

#include <stdio.h>

int main (int argc, char **argv)

{
        int i;
        int value;
        int total;
        char operator;


        total=atoi(argv[1]);

        for(i=2; argc>i+1; i=i+2)
        {        
                operator=argv[i][0];
                value=atoi(argv[i+1]);

                if(operator=='+')
                {
                           total=total+value;
                }        
                else if(operator=='-')
                {                
                           total=total-value;
                }
                else if(operator=='*')
                {                
                        total=total*value;
                }                
                else if(operator=='/')
                {
                        total=total/value;
                }

        }
        fprintf(stdout, "%d\n", total);

return 0;
}

Now I need to make it accept Roman numerals. I know i have to program the atoi function, but I dont know how to do that. It only need to go up to 4999. and when it prints the answer in roman numerals it can print like 456=CCCCLVI.
Can someone please help me? or at least give me a hint so I can do it myself
Thanks