From the enumeration of corners.

Terms

a(0) =0a(1) =2a(2) =110a(3) =2002a(4) =20020a(5) =136136a(6) =705432a(7) =2984520a(8) =10786908a(9) =34370050a(10) =98768670a(11) =260390130a(12) =638110200a(13) =1468635168a(14) =3200871520a(15) =6650874912a(16) =13248113736a(17) =25415833170a(18) =47143878782a(19) =84832157410a(20) =148507792972a(21) =253549890440a(22) =423093671000a(23) =691331713800