using System;

public class Program

{

    public static void Main()

    {

            int a;

            Console.WriteLine("Nhap a:");

            a = int.Parse(Console.ReadLine());

            int i = 1;

            int s = 1;

            while (i <= a)

            {

                s = s + i;

                i = i + 1;

            }

            Console.WriteLine($"Tong cua s la:{s}");

    }

}