RosettaCodeData/Task/Factorial/Processing/factorial-2.processing

long fi(int n) {
    if (n < 0){
      return -1;
    }

    if (n == 0){
      return 1;
    } else {
      long r = 1;

      for (long i = 1; i <= n; i++){
        r = r * i;
      }

      return r;
    }
}