Twice the large Schroeder numbers A006318.

Terms

a(0) =2a(1) =4a(2) =12a(3) =44a(4) =180a(5) =788a(6) =3612a(7) =17116a(8) =83172a(9) =412196a(10) =2075436a(11) =10586892a(12) =54595476a(13) =284157492a(14) =1490774076a(15) =7875206076a(16) =41854313412a(17) =223636052036