Killer sudoku keeps the sudoku rule — every row, every column and every three-by-three box holds the digits 1 to 9 once each — but starts with no printed digits at all. Instead the grid is divided into groups of neighbouring squares, marked by dashed outlines; the small number in a group’s top-left corner is the total its digits must add up to, and no digit may repeat inside a group.

A way in: every row, column and box adds up to 45, so where groups cover all but one square of a row, column or box, that square is 45 minus their totals. Tap a square, then type a digit or use the keypad below the grid.