Combining multiples and creating a range of numbers
 

I have a list of data that looks something like this:
1 Cats
2 Cats
3 Dogs
4 Dogs
5 Dogs
6 Birds
7 Birds

And I want to find a way to combine multiples and have the numbers become a range, like this:
1-2 Cats
3-5 Dogs
6-7 Birds

Here's an example of what I mean: https://docs.google.com/spreadsheet/...1TnNwdlgyR Wc

I would do it by hand, but there's 45,000 rows so it would be pretty time consuming. Any ideas?