Killer Sudoku combines classic sudoku with cage sum puzzles — also called sumdoku. The 9×9 board starts empty: there are no given digits. Instead, dashed outlines group cells into cages, and a small number in the top-left of each cage is the sum of the digits that belong there. Fill every cell with 1–9 so each row, column, and 3×3 box contains each digit once (the usual sudoku rules), and so the digits inside every cage add up to that cage’s sum without repeating. Tap a cell, then enter a digit with the pad or the 1–9 keys; arrows move the selection, and Backspace or the erase key clears a cell. Conflicting repeats in a row, column, box, or cage light up red immediately.

Strategy tips: start with small cages. A 3-cage summing to 6 must be 1-2-3; a 2-cage summing to 17 is 8-9. Cross those forced sets with the box and row constraints to place digits early. Watch for “innies and outies” — when a cage spills across a box, the portion inside often has only one possible sum left. Because digits cannot repeat inside a cage, larger cages rule out many candidates at once. Use Check anytime to compare your entries with the solution, and New game for a fresh set of cages. Every puzzle is generated from a valid sudoku solution, so it is always solvable. Beat the clock — your best score rises the faster you finish.