From the enumeration of corners.

Terms

a(0) =0a(1) =2a(2) =28a(3) =168a(4) =660a(5) =2002a(6) =5096a(7) =11424a(8) =23256a(9) =43890a(10) =77924a(11) =131560a(12) =212940a(13) =332514a(14) =503440a(15) =742016a(16) =1068144a(17) =1505826a(18) =2083692a(19) =2835560a(20) =3801028a(21) =5026098a(22) =6563832a(23) =8475040a(24) =10829000a(25) =13704210a(26) =17189172a(27) =21383208a(28) =26397308a(29) =32355010