As with Sudoku, the objective of Killer Sudoku is to fill the grid with the numbers 1 to 9, such that each row, column and box contains each number only once. Additionally, a Killer Sudoku grid is divided into cages, each cage has its own background color. The values of the cells in a cage must sum up to the total for that cage, shown in the cage's top left corner. The same number cannot appear in a cage more than once.