The most common version of Sudoku is composed by a square grid of 9 rows and 9 columns for a total of 81 cells. The grid is subdivided into nine 3×3 blocks or subgrids. Some of the 81 cells are filled in with numbers from 1 to 9. These filled-in cells are called givens. The goal is to fill in the whole grid using the nine digits so that each row, each column, and each block contains each number exactly once.
Usually, the initial grid is partially completed and a good sudoku should have an unique solution.