static void Main(string[] args)
{
//init singleton
var terminal = new PointOfSaleTerminal();
//singleton cart
//var terminal = PointOfSaleTerminal.Instance();
//add product code if Code not exit will print product not found, accept string or char
terminal.ScanProduct("CCCCCCCCCCCC");
//cacualte the result
decimal result = (decimal)terminal.CaculateTotal();
//write result to console
Console.Write("${0:N2}", result);
}
