My e-friend and coauthor, Konstantin Knop, designed the following problem for the 2011 All-Russia Olympiad:
Some cells of a 100 by 100 board have one chip placed on them. We call a cell pretty if it has an even number of neighboring cells with chips. Neighbors are the cells that share a side. Is it possible for exactly one cell to be pretty?
The problem is not easy. Only one person at the Olympiad received full credit for it.Share: