Hashi rule
They must begin and end at distinct islands, travelling a straight line in between.
They must not cross any other bridges or islands.
They may only run orthogonally.
At most two bridges connect a pair of islands.
The number of bridges connected to each island must match the number on that island.
Hashiwokakero (from Japanese "Hashi o kakero"; literally "build bridges"; also known as "Hashi", "Bridges", "Chopsticks" or "Ai-Ki-Ai") is a type of logic puzzle. It is played on a rectangular grid with no standard size, although the grid itself is not usually drawn. Some cells start out with (usually encircled) numbers from 1 to 8 inclusive; these are the islands. The rest of the cells are empty.