Signature

Chef bought an electronic board and pen. He wants to use them to record his clients' signatures. The board is a grid with $N$ rows (numbered $1$ through $N$) and $M$ columns (numbered $1$ through $M$) of pixels. Initially, all pixels are white. A client uses the electronic pen to sign on the board; whenever the pen touches a pixel, this pixel becomes black. Note that a signature may be non-cont