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

Terms

a(0) =5a(1) =22a(2) =103a(3) =504a(4) =2505a(5) =12506a(6) =62507a(7) =312508a(8) =1562509a(9) =7812510a(10) =39062511a(11) =195312512a(12) =976562513a(13) =4882812514a(14) =24414062515a(15) =122070312516a(16) =610351562517