Blacken exactly 2 cells in every row and column. Black cells cannot touch each other, not even diagonally. External numbers indicate how many white cells are between the two black cells in that row or column.
MOUSE: Click on a cell to blacken it. Click again to mark it empty. A third click will eliminate any symbol.
KEYBOARD: Press 'b' to blacken a cell. Press '-' to mark it empty. Press DEL to eliminate any symbol.