At the beginning of the game, each cell of a grid contains a number (or letter).
The goal is to color cells so that there is only one single uncolored number per row and per column.
The coloured cells must not touch each other from the sides. But their corners can touch each other.
All uncolored cells must be connected by their sides to form a single piece
(it must be possible to go through all the uncolored cells from one side to the other).