(n)! + (n + 1)!!/(n + 1) - 2.

Terms

a(0) =0a(1) =0a(2) =1a(3) =6a(4) =25a(5) =126a(6) =733a(7) =5086a(8) =40423a(9) =363262a(10) =3629743a(11) =39920638a(12) =479011993a(13) =6227066878a(14) =87178426333