using System;

​

public class Program

{

    public static void Main()

    {

        int n, count = 1, abc = 1;

        Console.WriteLine("enter a number: ");

        n = int.Parse(Console.ReadLine());

        while (count <= n)

        {

            abc = abc * count;

            count++;

        }

​

        if (n % 2 != 0)

            abc = abc * -1;

        Console.WriteLine("num" + abc);

    }

}