How do I recursively move and rename multiple files in multiple folders?

I would prefer a method of doing this in Linux, but Windows would be acceptable.

Suppose I have one folder "documents", containing three folders, A, B,C, each containing three files, 1.txt, 2.txt, 3.txt.

I want to move and rename all the files into "documents", such that the names are A-1.txt, A-2.txt, A-3.txt, B-1.txt, B-2.txt, etc.

How can I do this?

Thanks