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.