function GCD(x, y) {
  if (y == 1) return 1;

  for (let i = 2; i <= y; i++) {
    if (x % i == 0 && y % i == 0) {
      //divide our numbers
      x /= i;
      y /= i;

      return i * GCD(x, y);
    }

    //bottom of the recursion
    if (i == y) {
      return 1;
    }
  }
}