a(n) = n!/2-(-2)^(n-2)*(n-2).

Terms

a(0) =1a(1) =0a(2) =1a(3) =5a(4) =4a(5) =84a(6) =296a(7) =2680a(8) =19776a(9) =182336a(10) =1812352a(11) =19963008a(12) =239490560a(13) =3113532928a(14) =43589096448a(15) =653837290496