intresting estimate, hard example
Source: III Caucasus Mathematical Olympiad
March 17, 2018
combinatorics
Problem Statement
In the cells of an board, marbles are placed one by one. Initially there are no marbles on the board. A marble could be placed in a free cell neighboring (by side) with at least three cells which are still free. Find the greatest possible number of marbles that could be placed on the board according to these rules.