# A006157 (b-file synthesized from sequence entry)
2 1
3 5
4 28
5 180
6 1320
7 10920
8 100800
9 1028160
10 11491200
11 139708800
12 1836172800
13 25945920000
14 392302310400
15 6320426112000
16 108101081088000
17 1956280854528000
18 37347179950080000
19 750144785854464000
20 15813863053148160000
21 349121438173347840000