Sudoku is a logic-based combinatorial number-placement puzzle. In classic sudoku, the objective is to fill a 9X9 grid with digits so that each column, each row, and each of the nine 3X3 subgrids that compose the grid contain all of the digits from 1 to 9. The puzzle setter provides a partially completed grid, which for a well-posed puzzle has a single solution. Completed games are always an example of a Latin square, including an additional constraint on the contents of individual regions. For example, the same single integer may not appear twice in the same row, column, or any of the nine 3X3 subregions of the 9X9 playing board.