static void Example(int a, int b)
        {
            DuoNumbers duo = new DuoNumbers(a, b);

            duo.PrintAll();
            Console.WriteLine("\n\n");
        }
        static void Example2(int a, int b)
        {
            DuoNumbers duo = new DuoNumbers(a, b);

            duo.PrintDiv();
            duo.PrintAdd();
            duo.PrintAll();
        }