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

namespace ConsoleApplication25
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine()), Sum=0;
            if (Math.Abs(N)<10000)
            {
                while (N != 1)
                {
                    if (N > 1)
                    {

                        Sum = Sum + N;
                        N--;


                    }
                    else
                    {
                        Sum = Sum + N;
                        N++;


                    }
                }
            }
            Console.WriteLine(Sum+1);
        }
    }
}