Put the numbers 1 to 9 in the circles, each once. The number in the middle of a square is what its four corner circles must add up to; the middle circle is a corner of all four squares and is given. Tap a circle, then a number; tap the chosen circle again to let it go. A number that is already placed moves to the chosen circle, and if that circle had a number the two swap. To take a number out, tap its circle and then its number again. Tap a square's target to light up its four corners and work inside that square. Hint places one right number and counts as help.
Keys: 1–9 place, arrows move, Backspace clears, Z undo, Y redo, H hint.