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

Terms

a(0) =4a(1) =15a(2) =72a(3) =420a(4) =2880a(5) =22680a(6) =201600a(7) =1995840a(8) =21772800a(9) =259459200a(10) =3353011200a(11) =46702656000a(12) =697426329600