using System;

public class Program

{

    public static void Main()

    {

​

        int n;

​

​

​

        do{

            Console.Write("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);

        //Console.WriteLine("");

    }

}