Given a non-empty String and an Int N, return the String made starting with Char 0,
and then every Nth Charof the String.
So if N is 3, use char 0, 3, 6, ... and so on. N is 1 or more.
For this problem we're expecting only a method.
Don't define a class, include modifiers like static or public, add import statements, or add code outside the method declaration.