extended gcd

int gcd (int a, int b, int &x, int &y) {
    if (a == 0) {
        x = 0; y = 1;
        return b;
    }
    int x1, y1;
    int d = gcd (b % a, a, x1, y1);
    x = y1 - (b / a) * x1;
    y = x1;
    return d;
}