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2019 Greece National Olympiad Q4

Source: 2019 Greece National Olympiad

March 11, 2019
combinatorics

Problem Statement

Given a n×mn\times m grid we play the following game . Initially we place MM tokens in each of MM empty cells and at the end of the game we need to fill the whole grid with tokens.For that purpose we are allowed to make the following move:If an empty cell shares a common side with at least two other cells that contain a token then we can place a token in this cell.Find the minimum value of MM in terms of m,nm,n that enables us to win the game.