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

namespace timus
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            int s = 0;
            for (int i = 1; i <= Math.Abs(n); i++)
            {
                s += i;
            }
            if (n >= 1 && n <= 1000) { Console.WriteLine(s); }
            else if (n <= 0 && n >= -1000) { Console.WriteLine(-(s - 1)); }
        }
    }
}