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);