a(n) = 25*(5^n - 1)/4.

Terms

a(0) =0a(1) =25a(2) =150a(3) =775a(4) =3900a(5) =19525a(6) =97650a(7) =488275a(8) =2441400a(9) =12207025a(10) =61035150a(11) =305175775a(12) =1525878900a(13) =7629394525a(14) =38146972650a(15) =190734863275a(16) =953674316400