Liczby podzielne przez 3 w C#

 string k ;
            do
            {
            int a = 0, b = 0;
            int i, l = 0;
            Console.Clear();
            Console.WriteLine("Podaj pierwsza liczbe: ");
            a = Int32.Parse(Console.ReadLine());
            Console.WriteLine("Podaj druga liczbe: ");
            b = Int32.Parse(Console.ReadLine());
                if (b > a)
                {
                    if (a < 0)
                    {
                        a = 0;
                    }
                    else
                    {
                        for (i = a; i <= b; i++)
                        {
                            if (i % 3 == 0)
                                l += 1;
                        }
                    }
                }
                else
                {
                    if (b < 0)
                    {
                        b = 0;
                    }
                    else
                    {
                        for (i = b; i <= a; i++)
                        {
                            if (i % 3 == 0)
                                l += 1;
                        }
                    }
                }
                Console.WriteLine(l);
                Console.WriteLine("Aby zakonczyc program nacisnij n :");
                k = Console.ReadLine();
            }
            while (k != "n");