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;
					
public class my
{
	public static void Main()
	{
		var n = my.GetInt();
		Console.WriteLine(my.Factrial(n));
	}
	
	private static long Factrial(long n)
	{
		if(n==1)	return 1;
		else return n * my.Factrial(n-1);
	}

	
	private static int GetInt()
	{
		return int.Parse(Console.ReadLine());
	}
	
		private static int[] GetIntArray()
	{
		return Array.ConvertAll(Console.ReadLine().Split(' '), int.Parse);
	}

	private static int[] GetIntArray(int n)
	{
		var A = new int[n];
		var s = Console.ReadLine().Split(' ');

		for(int i = 0; i < n; i++) {
			A[i] = int.Parse(s[i]);
		}
		return A;
	}

	private static int[] GetIntArray(char c)
	{
		return Array.ConvertAll(Console.ReadLine().Split(c), int.Parse);
	}
	
	public static string Reverse(string s)
	{
		var c = s.ToCharArray();
		Array.Reverse(c);
		
		return new String(c);
	}
	
	private static void WriteArray(int[] a)
	{
		Console.WriteLine(string.Join(" ", Array.ConvertAll( a, x=>x.ToString() ) ) );
	}

	private static void WriteArray(long[] a)
	{
		Console.WriteLine(string.Join(" ", Array.ConvertAll( a, x=>x.ToString() ) ) );
	}
}