A006318(n) - 1.

Terms

a(0) =0a(1) =1a(2) =5a(3) =21a(4) =89a(5) =393a(6) =1805a(7) =8557a(8) =41585a(9) =206097a(10) =1037717a(11) =5293445a(12) =27297737a(13) =142078745a(14) =745387037a(15) =3937603037a(16) =20927156705a(17) =111818026017a(18) =600318853925