Term in Recamán's sequence A005132 where n appears for first time, or -1 if n never appears.

Terms

a(0) =0a(1) =1a(2) =4a(3) =2a(4) =131a(5) =129a(6) =3a(7) =5a(8) =16a(9) =14a(10) =12a(11) =10a(12) =8a(13) =6a(14) =31a(15) =29a(16) =27a(17) =25a(18) =23a(19) =99734a(20) =7a(21) =9a(22) =11a(23) =13a(24) =15a(25) =17a(26) =64a(27) =62a(28) =60a(29) =58