(8*5^n - 5*2^n)/3.

Terms

a(0) =1a(1) =10a(2) =60a(3) =320a(4) =1640a(5) =8280a(6) =41560a(7) =208120a(8) =1041240a(9) =5207480a(10) =26039960a(11) =130204920a(12) =651034840a(13) =3255194680a(14) =16276014360a(15) =81380153720a(16) =406900932440