-------------------------------------
| _   1   2 | 5   _   4 | 8   7   _ |
| _   4   _ | _   _   _ | _   _   _ |
| 7   5   _ | _   1   6 | 4   2   3 |
-------------------------------------
| 5   _   4 | 1   _   8 | 7   3   _ |
| _   2   _ | _   5   _ | _   4   _ |
| _   _   3 | 4   _   9 | 5   _   _ |
-------------------------------------
| 4   8   _ | _   _   5 | _   1   7 |
| _   _   _ | _   _   _ | _   _   _ |
| 2   3   5 | 7   _   1 | 6   9   _ |
-------------------------------------