int gcd(int a, int b)
{
    if((a==0) || (b==0))
        return 0; // to only check numbers if they are different from 0
    else if((a < 0) || (b < 0))
        return 0;

    do
    {
        while(a<b)
            b - a;
        while(b<a)
            a - b;
    }
    while(a!=b);
    return a;
}