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| n | a(n) |
|---|---|
| 0 | -1 |
| 1 | -1 |
| 2 | 0 |
| 3 | 3 |
| 4 | 12 |
| 5 | 40 |
| 6 | 130 |
| 7 | 427 |
| 8 | 1428 |
| 9 | 4860 |
| 10 | 16794 |
| 11 | 58784 |
| 12 | 208010 |
| 13 | 742898 |
| 14 | 2674438 |
| 15 | 9694843 |
| 16 | 35357668 |
| 17 | 129644788 |
| 18 | 477638698 |
| 19 | 1767263188 |
| 20 | 6564120418 |
| 21 | 24466267018 |
| 22 | 91482563638 |
| 23 | 343059613648 |
| 24 | 1289904147322 |
| 25 | 4861946401450 |
| 26 | 18367353072150 |
| 27 | 69533550916002 |
| 28 | 263747951750358 |
[-1,-1,0,3,12,40,130,427,1428,4860,16794,58784, 208010,742898,2674438,9694843,35357668,129644788, 477638698,1767263188,6564120418,24466267018, 91482563638,343059613648,1289904147322, 4861946401450,18367353072150,69533550916002, 263747951750358] |