using System;

public class Program

{

    public static void Main()

    {

        int n;

        do{

            Console.WriteLine("กรุณาใส่ตัวเลข 1-100 : ");

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

        }while(n<1 || n>100);

        int sum=0, i;

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

            sum += i;

        }

        Console.WriteLine("sum of 1-{0} : {1}",n,sum);

    }

}