This is Amongst the reasons I like the command line so much is that there are so many ways of doing something, almost all of which are easier than doing the same thing the WIMPy way, and some of them are so elegant.

I'm not sure of the strict requirements for the numbers, I just know that natural numbers work and they don't need to be sequential (although my second solution will make them so) or start at any number in particular. Neither does mine :)