static char GetMenuItem()
        {
            char menuItem;

            DisplayMenu();

            try
            {
                menuItem = char.Parse(Console.ReadLine());
                menuItem = char.ToUpper(menuItem);

                while (menuItem != 'E' && menuItem != 'V' && menuItem != 'X')

                {
                    Console.WriteLine("\nInvalid menu item!\n");
                    DisplayMenu();
                    menuItem = char.Parse(Console.ReadLine());
                    menuItem = char.ToUpper(menuItem);
                }
            }

            catch (Exception)
            {
                Console.WriteLine("\nInvalid menu item format!");

                menuItem = char.ToUpper(GetMenuItem());
            }

            return menuItem;
        }


static decimal GetPositiveDecimal(string message)
        {
            decimal getPositiveDecimal;

            Console.WriteLine(message);

            try
            {
                getPositiveDecimal = decimal.Parse(Console.ReadLine());

                while (getPositiveDecimal <= 0)
                {
                    Console.WriteLine("\nInvalid response - please enter a positive number.");

                    Console.WriteLine(message);
                    getPositiveDecimal = decimal.Parse(Console.ReadLine());
                }
            }

            catch (Exception)
            {
                Console.WriteLine("\nInvalid input number format!");

                getPositiveDecimal = GetPositiveDecimal(message);
            }

            return getPositiveDecimal;
        }


static double GetPositiveDouble(string message)
        {
            double getPositiveDouble;

            Console.WriteLine(message);

            try
            {
                getPositiveDouble = double.Parse(Console.ReadLine());

                while (getPositiveDouble <= 0)
                {
                    Console.WriteLine("\nInvalid response - please enter a positive number.");

                    Console.WriteLine(message);
                    getPositiveDouble = double.Parse(Console.ReadLine());
                }
            }

            catch (Exception)
            {
                Console.WriteLine("\nInvalid input number format!");

                getPositiveDouble = GetPositiveDouble(message);
            }

            return getPositiveDouble;
        }