Delete directories unless they have a certain file.

We've got lots of directories containing files that are important for a short time but then most become unneeded after a few months. We can't change the directory names so were wanting to mark the ones that need saved by adding a "save.txt" file.

I've been having trouble coming up with a command to delete all the directories and the contents unless they have the save.txt file. If they have the save.txt file we need to keep the directory and it's contents.

Any ideas?

Thanks in advance,
Tim