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 Volume0_0019
{
	class Program
	{
		static void Main(string[] args)
		{
			ulong n = ulong.Parse(Console.ReadLine());
			ulong a = 1UL;

			for(ulong i = 2; i <= n; i++) {
				a *= i;
			}

			Console.WriteLine(a);
		}
	}
}