I64 Factorial(I64 n) {
  if (n == 0)
    return 1;
  if (n < 0)
    return -1 * ((-1 * n) * Factorial((-1 * n) - 1));
  return n * Factorial(n - 1));
}

Print("+1:  %d\n", Factorial(1));
Print("+10: %d\n", Factorial(10));
Print("-10: %d\n", Factorial(-10));