I have checked all known numbers - result is right. When I submit code, system writes: Wrong answer. Where is error? Thank You!

using System;

class Program
{
    static void Main()
    {
        int a = int.Parse(Console.ReadLine());
        int r = 0;
        if (a <= System.Math.Abs(10000))
        {
            if (a > 1)
            {
                for (int i = 1; i <= a; i++)
                {
                    r += i;
                }
            }
            if (a < 1)
            {
                for (int i = 1; i >= a; i--)
                {
                    r += i;
                }
            }
            Console.WriteLine(r);
        }
        else
            throw new Exception("Wrong number");
    }
}

Hi,
how about the case when a = 1 ? It seems like you'll output 0 which is incorrect.

Thank you! That was an error.