using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Enter a number");

        int a=0;

        int total=1;

        string b= Console.ReadLine();

        bool c = int.TryParse(b, out a);

        if(c)

        {

            while(a>0)

            {

                total=total*a;

                a--;

            }

            Console.WriteLine(total);

        }

        else

        {

            Console.WriteLine("wrong input");

        }

    }

}