using System;

public class Program

{

    public static void Main(string[] args)

    {

        int n = 0;

        do

        {

            Console.Write("nhap so nguyen duong n:");

            n = Convert.ToInt32(Console.ReadLine());

    } while (n<1);

        uint sum = 0;

        for (uint i =1 ; i <= n;i++) 

        {

            sum += i;

        }

        Console.WriteLine("S({0}) = {1}" ,n, sum);

        Console.ReadLine();

    }