transparency - Get difference between two images as transparent image in GIMP

I have two images A and B which are similar in many regions.

I want to create an image C so that

B = C on top of A

where C should be as transparent as possible. on top of is the normal layer superposition.

So C is some kind of difference between A and B. If A and B are identical, then C is totally transparent. If A and B are very different, then their C would be nearly identical to B.