using System;

​

public class Program

{

    public static void Main()

    {

        int num,num2=-2, ex = 1, count = 1;

        Console.WriteLine("enter a number");

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

        while (count < num)

        {

            ex = ex * num2;

            if (num2>0)

            num2 = (num2 + 1) * -1;

            else 

            num2 = (num2 - 1) * -1; 

            count++;

        }

            Console.WriteLine(ex);

    }

}