*-----------*
|..4|.9.|...|
|.1.|...|..3|
|7..|.18|...|
|---+---+---|
|.8.|2..|.9.|
|539|...|726|
|.2.|..3|.1.|
|---+---+---|
|...|16.|..4|
|1..|...|.7.|
|...|.3.|8..|
*-----------*