using System;

public class Program

{

    public static void Main()

    {

        int n;

​

            Console.Write("Nhap vao n: ");

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

            if (n < 1)

            {

                Console.WriteLine("n khong thoa dieu kien");

            }

            else

            {

                int i = 1;

                int s = 0;

                while (i <= n)

                {

                    s += i;

                    i++;

                }

                Console.WriteLine($"Tong tu 1 den {n} la " + s);

            };

            Console.ReadLine();

    }

}