x x x  x 6 2  x x x
x 6 x  x x 9  3 x 8
4 x x  x x x  x 6 x

x 5 x  x x x  8 x x
x 8 x  4 1 5  x 3 x
x x 2  x x x  x 1 x

x 1 x  x x x  x x 3
2 x 3  5 x x  x 9 x 
x x x  6 9 x  x x x