Sudoku

The playing field is a grid with 9x9 cells, divided into smaller grids with a side of 3 cells. Thus, the entire playing field consists of 81 cells. At the beginning of the game, there are already some numbers in them (from 1 to 9). The player is required to fill in the free cells with numbers from 1 to 9 so that in each row, in each column and in each small 3 ×3 square, each digit would occur only once.
There are multiple levels of difficulty you can choose. Beware of mistakes you can make.