From the enumeration of corners.

Terms

a(0) =0a(1) =2a(2) =60a(3) =660a(4) =4290a(5) =20020a(6) =74256a(7) =232560a(8) =639540a(9) =1586310a(10) =3617900a(11) =7696260a(12) =15438150a(13) =29451240a(14) =53796160a(15) =94607040a(16) =160908264a(17) =265670730a(18) =427156860a(19) =670609940a(20) =1030350090a(21) =1552346268a(22) =2297341200a(23) =3344614000a(24) =4796473500