a(n) = (binomial(n,5) - floor(n/5)) / 5.

Terms

a(0) =1a(1) =4a(2) =11a(3) =25a(4) =50a(5) =92a(6) =158a(7) =257a(8) =400a(9) =600a(10) =873a(11) =1237a(12) =1713a(13) =2325a(14) =3100a(15) =4069a(16) =5266a(17) =6729a(18) =8500a(19) =10625a(20) =13155a(21) =16145a(22) =19655a(23) =23750a(24) =28500a(25) =33981a(26) =40274a(27) =47466a(28) =55650a(29) =64925