using System;

namespace ConsoleApplication
{
    internal class Program
    {
        public static void Main(string[] args)
        {
            var i = Convert.ToInt32(Console.ReadLine());
            var v = i;
            var b = v;
            var z = Math.Abs(i);
            var x = 0;
            if (b<0)
            {
                x = -2;
            }
            else
            {
                x = 0;
            }
            var sum = 0;
            while (x!=z)
            {
                sum = sum + v;
                if (b<0)
                {
                    v++;
                }
                else
                {
                    v--;
                }
                x++;
            }
            Console.WriteLine(sum);
        }
    }
}