using System;

namespace Consoleapp
{
    class Program
    {
        static void Main(string[] args)
        {
            int sum = 0;
            int len = Convert.ToInt32(Console.ReadLine());
            for (int i = 2; i <= len; i++)
            {
                sum = sum + i;
            }
            Console.WriteLine(sum);
        }
    }
}

Absolute value not exceeding 10000 means the number can be in range -10000..10000.