using System;

public class Program

{

    public static void Main()

    {

        int num=0, total=0, x=0;

        do

        {

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

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

        }while(num <0 || num > 100);

        while(x <= num){

            total += x;

            x++;

        }

​

        Console.WriteLine("\nSum of 0-{0} : {1}",num,total);

    }

}