Programming challenge #1. Breaking chocolate problem

I have been having a lot of fun with Codewars so I’ve decided to create a “Programming challenge” section to publish some coding challenges and their solutions (of course in a spoiler, don’t be naughty!) in order to check if you’re right when you are done. Usually I will solve them using Java or Python, but you can use what you want. Let’s start!

The problem

Bill is a fat and a lazy guy who loves eating chocolate, but due to his lazyness he wants to break the chocolate bar with the minimum number of breaks needed.

This is what he wants:

Your task is to split the chocolate bar of given dimension n x m into small squares. Each square is of size 1x1 and unbreakable. Implement a function that will return minimum number of breaks needed.

For example if you are given a chocolate bar of size 2 x 1 you can split it to single squares in just one break, but for size 3 x 1 you must do two breaks.

If input data is invalid you should return 0 (as in no breaks are needed if we do not have any chocolate to split). Input will always be a non-negative integer.