internal static void Run()
        {
            var menu = new Menu("Enter command: ");

            menu.Add("a", "Add ten", AddTen);
            menu.Add("b", "Multiply by four", MultiplyByFour);
            menu.Add("c", "Subtract by five", SubtractByFive);

            cc.DisplayMenu(menu);
            DisplayNumber();

            cc.DisplayMenu(menu);
            DisplayNumber();

            cc.DisplayMenu(menu);
            DisplayNumber();
        }