Sudoku rules
Place the numbers 1 to 9 in each cell of the 9x9 field the way that they repeat in neither columns, nor lines, nor 3x3 squares. The round is completed when each cell has a number in it.

The Principle
The game is played in difficulty levels: once you win one round, you progress to the next level. If you fail, you go one level down. The game starts by default at the medium difficulty level – 21.