using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("enter number");

        String s = Console.ReadLine();

        int num = 0;

        int res = 0;

        if(int.TryParse(s, out num))

        {

            num = Convert.ToInt32(s);

            res = 1;

            for(int i = 1; i <= num; i++)

            {

                res *= i;

            }

        }

        else

        Console.WriteLine("invalid");

        Console.WriteLine(res);

    }

}