private static decimal GetGrandTotalFrom(ShellViewModel viewModel)
        {
            if (string.IsNullOrWhiteSpace(viewModel.GrandTotal))
                return 0m;

            string grandTotalString = viewModel.GrandTotal.Substring(1);
            decimal grandTotalAmount;
            if (!decimal.TryParse(grandTotalString, out grandTotalAmount))
                return 0m;

            return grandTotalAmount;
        }
        private static ShellViewModel CreateViewModelWithBooks(int[] books)
        {
            var vm = new ShellViewModel();
            foreach (var book in books)
            {
                if (book == 1)
                    vm.Book1Quantity++;
                if (book == 2)
                    vm.Book2Quantity++;
                if (book == 3)
                    vm.Book3Quantity++;
                if (book == 4)
                    vm.Book4Quantity++;
                if (book == 5)
                    vm.Book5Quantity++;
            }

            vm.Calculate();

            return vm;
        }