a(n) = ceiling((n-3)(n-4)/6).

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =2a(5) =4a(6) =5a(7) =7a(8) =10a(9) =12a(10) =15a(11) =19a(12) =22a(13) =26a(14) =31a(15) =35a(16) =40a(17) =46a(18) =51a(19) =57a(20) =64a(21) =70a(22) =77a(23) =85a(24) =92a(25) =100a(26) =109a(27) =117a(28) =126a(29) =136