Kakuro is a crossword done with digits. Each run — a straight line of white squares, across or down — takes the digits 1 to 9 so that they add up to the number given for it, and no digit may be used twice within the same run. Those numbers stand in the dark squares at the head of each run: the total in the upper right belongs to the run going right, the one in the lower left to the run going down. Where a single dark square heads both, a diagonal separates the two. Tap a square, then type a digit or use the keypad below the grid.