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

Terms

a(0) =0a(1) =0a(2) =4a(3) =33a(4) =236a(5) =1795a(6) =15114a(7) =141113a(8) =1451512a(9) =16329591a(10) =199583990a(11) =2634508789a(12) =37362124788a(13) =566658892787