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2 player game of m tiles of kx1 on a nxn board k <= n <= 2k - 1

Source: 2020 Dutch IMO TST 3.4

November 22, 2020
combinatoricstilesTiling

Problem Statement

Given are two positive integers kk and nn with kn2k1k \le n \le 2k - 1. Julian has a large stack of rectangular k×1k \times 1 tiles. Merlin calls a positive integer mm and receives mm tiles from Julian to place on an n×nn \times n board. Julian first writes on every tile whether it should be a horizontal or a vertical tile. Tiles may be used the board should not overlap or protrude. What is the largest number mm that Merlin can call if he wants to make sure that he has all tiles according to the rule of Julian can put on the plate?