Fill the grid so that every row, every column and every three-by-three box contains the digits 1 to 9, each exactly once. The digits already printed are fixed; the rest is yours to work out. Every puzzle has exactly one solution, and it can always be reached by reasoning — you never have to guess.

A way in: find a row, column or box with a single empty square — the digit missing from it is the only one that can go there. Tap a square, then type a digit or use the keypad below the grid.