Series Game

            int a1, an, d, guess;
            Random rnd = new Random();
            while (true)
            {
                a1 = rnd.Next(1, 100);
                d = rnd.Next(-20, 20);
                Console.Write(a1 + " , ");
                an = a1 + d;
                for (int i = 0; i < 3; i++)
                {
                    Console.Write(an + " , ");
                    an += d;
                }
                guess = int.Parse(Console.ReadLine());
                if (guess == an)
                    Console.WriteLine("Correct!");
                else
                    Console.WriteLine("False! Next number is " + an);
                Console.ReadLine();
            }