Marutaringu

Marutaringu ("Maltese Ring") is a logic puzzle published by Nikoli. A square or rectangular grid is divided into regions. The goal is to blacken some cells of a grid according to the following rules:

• Every black cell must be orthogonally adjacent to exactly two other black cells.
• No 2 x 2 cell area within the grid can have the black color.
• A cell with a number indicates how many cells in the region must be blackened. A cell with a number also may be blackened.
• In regions without a number any amount of cells may be blackened (at least one).
• The black cells in a region must form a rectangle. A region contains exactly one black rectangle.

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