The grey cells in the centre form a magic square. Ie all rows, columns and the two diagonals each add to the same total. Inside each dotted box, all numbers have to be different and sum to the small clue given in the top left corner of the box.
The grey cells in the centre form a magic square. Ie all rows, columns and the two diagonals each add to the same total. Inside each dotted box, all numbers have to be different and sum to the small clue given in the top left corner of the box.