Kurotto

Kurotto (from Japanese, literally "black units") is a logic puzzle published by Nikoli. The task consists of a grid, with circles in some cells; a circle may have a number inside or not. The task is to blacken some cells of a grid according to the following rules:

• Cells with circles can not be blacken.
• Black cells form orthogonally contiguous areas.
• A number in a circle indicates the total number of black cells in areas orthogonally neighboring the circled cell.
• Cells with empty circles may have any number of black cells around them.

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