Sudoku Classic

Classical Sudoku is played over a 9x9 grid, divided to 3x3 sub grids.
Classical Sudoku begins with some of the grid cells already filled with numbers.
The object of Sudoku is to fill the other empty cells with numbers between 1 and 9.
Number can appear only once on each row, and number can appear only once on each column.
Also,number can appear only once on each 3x3 sub grids.
The difficulty can be selected by three stages.