Machine Gun

Country X has a military zone framed as a grid of size MxN. Each point of the grid could be free, occupied by a machine gun or by a protector. A machine gun at point (x, y) could automatically attack other guns at points (x-2, y-2), (x-2, y+2), (x+2, y-2) and (x+2, y+2). So we could not put two guns at points which make them destroy each others, unless there is a protector exactly in the middl