Практикум 4, задание 2, номер 11

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace mainSolution
{
    public class MyException : Exception
    {
        public MyException(string messege) : base(message: messege + "\n\n\n Exception founded by Decibit\n\n\n") { }
        public MyException() : base()
        {

        }
        public void Messsage()
        {
            Console.WriteLine("\n\n\n\n\n\n Invalid number, please enter a number from 1 to 12 \n\n\n\n\n\n");
        }


    }
    class Input
    {
        private static IEnumerator<string> getin()
        {
            while (true)
                foreach (string s in Console.ReadLine().Split().Where(x => x.Length > 0))
                    yield return s;
        }

        private IEnumerator<string> inp = getin();

        public string GetString() { inp.MoveNext(); return inp.Current; }
        public int GetInt() { return int.Parse(GetString()); }
        public long GetLong() { return long.Parse(GetString()); }
        public double GetDouble() { return double.Parse(GetString()); }
    }

    static class Program
    {
        static public Input cin = new Input();


        static void Main(string[] args)
        {
            int n = cin.GetInt();
            long sum = 0;
            long fact = 1;
            int a = 1;
            for (int i = 1; i <= n; ++i)
            {
                fact *= i;
                sum += a * fact;
                a = -a;
            }
            Console.WriteLine(sum);
        }

    }
}