Killer Sudoku: Number Puzzle

Killer sudoku: a sudoku grid with dashed cages drawn over it, each carrying the
total its squares must add up to, which is why it is also called sum sudoku.
Easy boards start you off with numbers on the grid. Expert gives you nothing but
the cages.

Fill the grid so every row, column and box holds 1 to 9 once each, every cage
adds up to its total, and no digit repeats inside a cage.

AS MUCH OF A HEAD START AS YOU WANT

Easy boards hand you around twenty numbers to build on. Medium gives you a
dozen, hard gives you a handful, and expert gives you an empty grid and nothing
but the cages, which is what killer sudoku is famous for. Every layout was
proved to pin exactly one answer before it shipped; a puzzle with two solutions
was never built.

THE DIFFICULTY IS MEASURED, NOT GUESSED

Most puzzle apps set difficulty by how big the cages are, which is a bad
measure. Every puzzle here was solved the way a person solves one, one technique
at a time, and graded by the hardest technique it needed.

Easy Cage totals and scanning. Two squares totalling 4 can only be 1 and 3.
Medium Where a digit cannot go, and cages that lock a digit into a box.
Hard The rule of 45, and pairs of squares that use up two digits.
Expert Triples, on a grid with no numbers given at all.

HINTS THAT TEACH INSTEAD OF TELLING

Ask for a hint and it names the rule. "This cage needs 4 more across two
squares, and 4 splits only as 1 and 3." Not "the answer is 3". Every puzzle can
be finished by reasoning alone, with no guessing anywhere, because a puzzle the
hint engine could not explain was never shipped.

A GUIDED FIRST PUZZLE

The first board walks you through it one rule at a time, on the board rather
than in a card you tap away. It teaches what the dashes mean, what the small
number in the corner means, and why a cage never repeats a digit. If you have
never seen a killer sudoku, start there.

WHAT IS IN IT

Hundreds of puzzles across four difficulties
A new puzzle every day, the same one for everybody, with a streak
Pencil marks, with the auto-clear a killer sudoku actually needs
Mistake checking that finds the pencil marks you have poisoned, not just the
digits you got wrong
Colours to earn for your ink, your paper and your highlight
Boosters if you want the help, paid for with nibs you earn by playing
Play offline. Every puzzle is in the app.

OPTIONAL PURCHASES

Packs of nibs and a booster bundle are for sale, plus a one-time starter set.
A one-off purchase removes the banner and the advert between puzzles. Every AD
button keeps working after any purchase. No puzzle is ever locked behind one.

THE ADVERTS, IN FULL

A banner sits at the foot of the puzzle screen and nowhere else, in a strip
kept clear for it so the grid never moves under a finger. It waits until you
have solved a puzzle or played for three minutes.

A short full-screen advert may play when you move on from a solved puzzle,
never in the middle of one: none before your third, then at most one every
second puzzle, 90 seconds apart at the least, none in the first minute of a
session and ten a session at the most.

Every other advert is a button you choose to press. It says AD first and names
what it gives you: a hint, a square filled in, a check of your grid, your board
kept after three mistakes, or double the pencils you were just paid. Starting
the same puzzle again is always free.

No energy. No lives to wait for. No account.