using System;
namespace gholamali
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                int N = Convert.ToInt32(Console.ReadLine());
                if (Math.Abs(N) <= 10000)
                {
                    if (N > 1)
                        Console.Write(((N + 1) / 2) * N);
                    else
                        Console.Write(((N + 1) / 2) * (2 - N));
                }

                Console.ReadKey();
            }
            catch { }
         }
    }
}

[TestCase("2", "3")]

((N + 1) / 2) * N
when you divide on int (= 2) it produce int result (= 1) and total sum is wrong (= 2)

(N + 1) / 2) * N
First multiply and then divide by 2

Like (N + 1) *N) / 2
Otherwise it may not act like an integer.
and for second case
((N + 1) * (2 - N)) / 2

Edited by author 04.10.2016 22:27