2^(2n-6) * C(n,3) - 2^(n-2) * C(n,4).

Terms

a(0) =1a(1) =12a(2) =120a(3) =1040a(4) =7840a(5) =52864a(6) =327936a(7) =1912320a(8) =10644480a(9) =57164800a(10) =298428416a(11) =1522626560a(12) =7622451200a(13) =37551144960a(14) =182458122240a(15) =875972788224