Apply sudoku rules. The puzzle is toroidal and the left/right and top/bottom edges of the grid map to each other. The digits 1 to 9 will appear once in each of the 10 rows, 10 columns, and 10 3×3 regions. The outer-edge of the grid (where the repeat occurs) is light gray/dark gray to help identify the wrapping. Note that the grid contains some black squares which will not contain any numbers.