int mod(int x, int m) {
    return (x%m + m)%m;
}

int mod(int x, int m) {
    int r = x%m;
    return r<0 ? r+m : r;
}

-13%2 = -1 so this might cause problem in your program sometime. So always write your code in such a way that it never cause any problem in sleep also.. :) Use anyone of the following code snippet.

Safety for checking MOD for negative values

Java