How to play Sudoku?

Sudoku is played on a square field 9 by 9 of cells. The field itself is divided into regions (3 by 3 squares). The goal of Sudoku is to fill all the empty cells with numbers 1-9 (one digit per cell), using the following rules:

1. The number can appear only once in each row.
2. The number can appear only once in each column.
3. The number can appear only once in each district.

Simply put, the same number can appear only once in each row, column and region.