In a board 8x8, the unit squares have numbers 1ā64, as shown. The unit square with a multiple of 3 on it are red. Find the minimum number of chess' bishops that we need to put on the board such that any red unit square either has a bishop on it or is attacked by at least one bishop.
Note: A bishops moves diagonally.
https://i.imgur.com/03baBwp.jpeg combinatoricsnational olympiad