using System;

​

public class Program

{

    public static void Main(string[] args)

    {

        Console.WriteLine("Введите n");

        double n = Convert.ToDouble(Console.ReadLine());

        int result = 1;

        for ( int chislo=2; chislo<n+1; chislo++)

        {

            result *= chislo;

        }

        result /=2;

        Console.WriteLine(result);

    }

    }

​