using System;

class FactorialProg{

  public static BigInteger Factorial(int n){
    require n >= 0;                                 
    BigInteger res = 1;                             
    for(int i = 1; i <= n; i++)                     
      res = res * i;
    return res;
    ensure res = n * n-1 * ... * 1;  
  }  

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

}