Rules: Shade some cells to divide the grid into groups (polyominoes) of unshaded cells called islands. Clue cells may not be shaded. Each island must contain exactly one clue cell, and the clue number must be equal to the size of the island. Additionally, the shaded cells must form a continuous, connected wall (a single polyomino) that contains no 2×2 squares of shaded cells.
This puzzle type was invented by Nikoli.