Problem:
Factorial

Write a program which reads an integer n and prints the factorial of n. You can assume that n ≤ 20.

Input

An integer n (1 ≤ n ≤ 20) in a line.

Output

Print the factorial of n in a line.

Sample Input

5

Output for the Sample Input

120

Solution in C# (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class AOJ
{
    static void Main(string[] args)
    {
        int n = int.Parse(Console.ReadLine());
        Console.WriteLine(fact(n));
    }
    static long fact(int n)
    {
        if (n == 0) return 1;
        return n * fact(n - 1);
    }
}