3
8 x x x x x x x x
x x 3 6 x x x x x
x 7 x x 9 x 2 x x
x 5 x x x 7 x x x
x x x x 4 5 7 x x
x x x 1 x x x 3 x
x x 1 x x x x 6 8
x x 8 5 x x x 1 x
x 9 x x x x 4 x x