using System;

​

public class Program

{

    public static void Main()

    {

        int k = Convert.ToInt32(Console.ReadLine());

        int n = 0;

        while (k > 0)

        {

            n++;

            k--;

        }

​

        int fib = 0;

        while (n > 0)

        {

            fib += n;

            n--;

        }

​

        Console.WriteLine(fib);

    }

}