using System;

public class Program

{

    public static void Main()

    {

            int firstnumber;

            int sum;

            int hropmerktUtkoma = 1;

            sum = 0;

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

        firstnumber = Convert.ToInt32(Console.ReadLine());

        if (firstnumber < 0)

            {

                Console.WriteLine("getur ekki hrópmerkt mínustölu");

                return;

            }

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

        {

            hropmerktUtkoma *= i+1;

        }

        if (firstnumber == 0)

        {

            Console.WriteLine("0");

            return;

        }

        else 

        {

        Console.WriteLine( + firstnumber + "! " + hropmerktUtkoma);

        }

    }       

​

}