Rules: Draw a single, non-intersecting loop running through some of the cells’ centers. The loop may not run diagonally: only up, down, left, or right. Cells not visited by the loop are shaded: two shaded cells may not share an edge (but touching by corners is allowed). The loop doesn’t run through clue cells, but they aren’t considered shaded either. A clue’s number tells how many shaded cells there are in the arrow’s direction. (Notice that there may be shaded cells not hinted at by any clue.)
Some puzzles use gray cells without a number and an arrow in them. These cells are like clue cells, meaning the loop doesn’t run through them, and they aren’t considered shaded either.
This puzzle type was invented by Nikoli.