I was trying to solve this problem here. In my initial submission here, I am going from a mask with only one fish to a complete mask, and it works. But in the case of this submission here, I am going from a complete mask to a particular fish, but apparently there is some overcounting in this case. Can someone please help me understand what is wrong in the second submission. Thanks in advance!

I am trying to solve this problem. The editorial for this problem uses the following approach

Can anyone explain me the Inclusion-Exclusion formula shown here?