Santoitchi

Santoitchi ("San to itchi"; from Japanese, literally "three and one") is a logic puzzle invented by Naoki Inaba (Japan). A rectangular or square grid contains numbers in some cells. The task is to blacken some empty cells according to the rules:

• All white cells must be divided on regions having the size of exactly three cells.
• Two black cells must not be orthogonally adjacent.
• Each region contains a number.
• A number indicates how many black cells share an edge with that region.

Cross+A can solve puzzles from 3 x 3 to 30 x 30.