static void Main(string[] args)
{
Rational rational = new Rational(1, 3);
Rational another = new Rational(1, 2);
rational.Add(another);
Console.WriteLine("Sum: " + rational.Numerator + "/" + rational.Denominator);
ShoppingCart cart = new ShoppingCart();
cart.AddProduct(1400);
cart.AddProduct(17);
cart.Checkout();
}