Block Drop

There is a square pool divided into NxM cells. In some cells of the pool there stone islands. Each island consists of some number of stones. Let's call this number the height of the island. You can jump from the island with coordinated (x,y) to any island with coordinates (x+1,y), (x+2,y), (x-1,y), (x-2,y), (x,y+1), (x,y+2), (x,y-1), (x,y-2). When you jump off the island its height goes down by on