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

namespace _1068
{
    class Program
    {
        static void Main(string[] args)
        {
            int number = int.Parse(Console.ReadLine());
            int result = 0;
            if (number > 10000 || number < -10000)
                throw new Exception("BAD!!!");
            if (number > 0)
                for (int i = 1; i <= number; i++)
                {
                    result += i;
                }
            if (number < 0)
                for (int i = 1; i >= number; i--)
                {
                    result += i;
                }
            Console.WriteLine(result);
        }
    }
}