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

namespace _1068
{
    class Program
    {
        static void Main(string[] args)
        {

            decimal a, sum = 0,i=0;
            a = Convert.ToInt32(Console.ReadLine());
            while (a<i)
            {
                i--;
                sum += i;
            }

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