Breaking up can be sweet...

I have just bought a beautiful block of chocolate. It is a normal rectangular-shaped block with 5 rows, and 4 pieces of chocolate to a row, making 20 pieces of chocolate ready for the eating. I want to eat all of it right now, but I want to savour each piece.

What is the least number of clean snaps necessary to break the block of chocolate into the 20 individual pieces?

What about for a block with n rows of m pieces?

If you are stumped by last issue's puzzle, here is the solution.

For some challenging mathematical puzzles, see the NRICH puzzles from this month or last month.