Solution illustrating a(6) = 14 by Antonio Misericordia:
+---+---+---+---+---+---+
| 2 | 7 | | 5 | 4 | 0 |
+---+---+---+---+---+---+
| 10| | | | 6 | 1 |
+---+---+---+---+---+---+
| | | | 11| | 3 |
+---+---+---+---+---+---+
| | | | 13| | 8 |
+---+---+---+---+---+---+
| | | | | | |
+---+---+---+---+---+---+
| | | 12| | 9 | |
+---+---+---+---+---+---+
|