Each cell in the grid represents a room. All rooms are interconnected by doors. Some doors are open, while the others are closed. The number inside a room indicates the number of rooms that can be seen from it in all four directions, including itself. Draw the closed doors.
(Rules and example from WPC 2017 IB)
History of the puzzle
Origin unknown. Probably of Czech origin, as it had appeared in WPC 2001.
Also called Open Office, Seethrough, Vista, etc.