Implement rules:
 Create a simple string calculator with a method int Add(string numbers).
 The method can take 0, 1 or 2 numbers, and will return their sum (for an empty string it will return 0) for example “” or “1” or “1,2”.
 Allow the Add method to handle an unknown amount of numbers.
 Allow the Add method to handle new lines between numbers (instead of commas).
 The following input is ok: “1\n2,3”
 The following input is NOT ok: “1,\n”