9=13→1-8-10-18
1-5-9-10
4-6-9- 11-12-14-17
2-7-10-11-12-16
2-5-6-14-15-18
15→6,10.11,14,17
1-4-6-7-8-13
4-7-8-13-14
2-4-9-11-12-15
1-2-4-5-9-17