using System;

class FactorialProg{

  public static BigInteger Factorial(int n){
    if (n >= 0){
      BigInteger res = 1;
      for(int i = 1; i <= n; i++)
        res = res * i;
      return res;
    }
    else throw new ArgumentException("n must be non-negative");
  }  

  public static void Main(){
    Console.WriteLine("{0}! = {1}", 5, Factorial(5));
  }

}