2n x 2n board with white/black markers
Source: All-Russian MO 2000
December 30, 2012
combinatorics unsolvedcombinatorics
Problem Statement
On some cells of a board are placed white and black markers (at most one marker on every cell). We first remove all black markers which are in the same column with a white marker, then remove all white markers which are in the same row with a black one. Prove that either the number of remaining white markers or that of remaining black markers does not exceed .