I was trying to solve the problem supposing that I have to choose a square subboard of the given chess board from either topleft side or bottomright side. But in the problem it is given:
So what exactly is said in the problem, by "arbitrarily chosen top left and bottom right cell", can anyone explain it with some example. Thanks asked 20 Oct '18, 18:06

say rectangle's top left=(1,0) and bottom right(5,5) now 4 verices of rectangle are TOP_LEFT(1,0) TOP RIGHT (1,5) BOTTOM LEFT (5,0) BOTTOM RIGHT (5,5) we can uniquely determine a rectangle by these 2 points. also topleft.x<=bottomright.x&&topleft.y<=bottomright.y answered 20 Oct '18, 21:53

it mean that chessboard size can be of any height, breath.