a(n) = C(n+2,3) + 2*C(n,2) + 2*(n-2).

Terms

a(0) =-1a(1) =6a(2) =18a(3) =36a(4) =61a(5) =94a(6) =136a(7) =188a(8) =251a(9) =326a(10) =414a(11) =516a(12) =633a(13) =766a(14) =916a(15) =1084a(16) =1271a(17) =1478a(18) =1706a(19) =1956a(20) =2229a(21) =2526a(22) =2848a(23) =3196a(24) =3571a(25) =3974a(26) =4406a(27) =4868a(28) =5361a(29) =5886