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;

namespace AOJ0019
{
    class Program
    {
        static void Main()
        {
            Console.WriteLine(Factorize(int.Parse(Console.ReadLine() ?? "")));
        }

        public static long Factorize(int n)
        {
            if(n < 0) throw new ArgumentException();
            if (n <= 1) return 1;
            return n*Factorize(n - 1);
        }
    }
}