2n*binomial(2n,n) - 4^n.

Terms

a(0) =-1a(1) =0a(2) =8a(3) =56a(4) =304a(5) =1496a(6) =6992a(7) =31664a(8) =140384a(9) =613016a(10) =2646544a(11) =11325200a(12) =48122528a(13) =203306736a(14) =854829344a(15) =3579783776a(16) =14939605184a(17) =62162742296a(18) =257985394064