a(n) = n!! - 1.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =7a(5) =14a(6) =47a(7) =104a(8) =383a(9) =944a(10) =3839a(11) =10394a(12) =46079a(13) =135134a(14) =645119a(15) =2027024a(16) =10321919a(17) =34459424a(18) =185794559a(19) =654729074a(20) =3715891199a(21) =13749310574a(22) =81749606399a(23) =316234143224