Stostone ("Stone on Stone") is a logic puzzle invented by Nikoli. A rectangular or square grid is divided into regions. The goal is to blacken some cells of a grid ("stones") according to the following rules:
- All black cells in a region must be connected horizontally or vertically.
- A cell with a number indicates how many cells in the region must be blackened.
- In regions without a number any amount of cells may be blackened (at least one).
- When two cells are orthogonally adjacent across a region boundary, at least one cell must be white.
- If all stones "fall down", they must cover exactly the bottom half of the grid.
Cross+A can solve puzzles from 4 x 4 to 30 x 30.