Please let me know if it is scientifically-valid to use Band Math in SNAP toolbox to subtract two binary images (0, 1) to get the difference between Pre- and Post flood events. ( 1 mean flood. 0 mean non-flood).

Actually I tried it and got difference map but I want to make sure if it is valid method.

I there should be 4 possible cases: (1) recorded flood events in the post- and pre-images and the result would be (1-1=0), (2) flood event in the post-image and no flood in the pre-image, and the result would be (1-0=1), (3) non-flood in the post-image and flood in the pre-image, and the result would be (0-1=-1), and non-flood in both post-and pre-images, so the result would be (0-0=0). The nain concern around the interpretation of the third case. Please adice