How can I improve comma insertion code?

[Cannibalope]

Here is the code I have written. It works fine except if the input is a negative six-digit number, a comma appears just after the negative sign like so: -,963,562

This is part of a larger program, obviously, and one of the other things I need to do is have negative numbers like -45 recognized as valid but numbers like 4-5 not recognized as valid. I hope this is enough information for someone to help me.

string commaString()
       {
            if (validInput = true);
            {
            string comString = inString;
            int length = comString.length();


            int i = (comString.length() - 3);
            while (i > 0)
            {
                comString.insert(i, 1, ',');
                i = i - 3;
            }


            return comString;
            }
        
       }