Killer Sudoku aren't just harder Sudoku puzzles! They're a distinct puzzle type that mixes elements of Sudoku and Kakuro puzzles.

### How to play

If you're new to Killer Sudoku puzzles, we've created a brief video guide which explains the basic techniques and walks you through solving a puzzle:

As with regular Sudoku, the objective of the puzzle is to fill the grid with the numbers 1-9 so that each row, column and nonet (3x3 group of squares*) contains each number exactly once.

Instead of prefilled squares acting as clues, the Killer Sudoku grid is divided into irregularly shaped sections (called 'cages'). Each cage contains a clue number in the corner of one of its squares: this number indicates the sum total of the numbers that should be entered in the cage's squares.

A completed cage must not contain any single number more than once.

Tap on a grid square to select it, then use the keypad buttons below the grid to enter numbers.

The top row of buttons enter numbers in pen (indicating that they're part of the final answer). The bottom row allow you to enter numbers in pencil, to mark which numbers are still valid for that square.

You can erase numbers you've entered by reselecting the square you want to edit, and using the keypad buttons to 'switch off' (erase) entered numbers.

*this implementation of Killer Sudoku also supports puzzles with smaller grids, with 2x2 and 3x2 nonets.

### The Rule of 45

When playing a full size (9x9) Killer Sudoku board, you can use the following technique:

All the numbers in a given row, column or nonet should add up to 45.

This rule can also be used smaller puzzles: the number is 21 for a 6x6 puzzle and 10 for a 4x4 puzzle.

Take a look at the Killer Sudoku grid below. Notice the top row contains a cage with the sum '8' entirely on the row covering three cells, and another cage with sum '5' covering the remaining cell in that top row and the one cell beneath.

Using the rule for a 4x4 grid, we know the row total is 10, so with the 8 given by the cage for the first three cells, it means the cell in the top right must be a 10 - 8 = 2; You can then you work out from the cage total being 5 that the cell underneath must be a 5 - 2 = 3 to complete it.

Now take the bottom row, - it contains a horizontal cage of two cells with a total value of 6 and another cage of 3 cells with a total value also of 6. So using the rule again, the two left most cells must total 10 - 6 = 4 ; we don't yet know the individual numbers to put in those two cells, but it does mean you can compute the third cell: 6 - 4 = 2, so the second cell along on row 3 is a 2.

This rule can also be applied to multiple rows, columns or nonets.

For the final example look at the two rightmost columns, all cells except one are contained within cages entirely in those columns.

Adding up all complete cage totals gives: 5 + 8 + 6 = 19 .

The total of two columns is 10 x 2 = 20, so the third cell along on the top row must can be deduced from 20 - 19 = 1.

### Extra help

If you're stuck on a Killer Sudoku puzzle, you can use coins to buy hints.

Hints can be used by tapping on the relevant icon in the status bar area at the top of the screen.

The cost (in coins) to use each hint is printed under its icon.

■ Solve Cell
Use this hint to put the correct number into this cell only.

■ Show Pencil Numbers
Use this hint to fill in all pencil marks showing which possible numbers are valid in that location.

■ Check for Errors
Use this hint to highlight all the cells with incorrect values.