The proposer Akiva Weinberger provided a twist on the classic “proof on the back of a napkin,” namely, a “solution using a napkin”:
In case it’s not quite clear how to rearrange the pieces, here are the “before” and “after” diagrams with the corresponding pieces numbered.
If you can find a way to do it with four or fewer pieces, that will be a new discovery indeed! (It may not be possible, but on the other hand, the Problem Warden is not aware of any proof of impossibility.)