First occurrence of -n in A220115.

Terms

a(0) =0a(1) =4a(2) =32a(3) =320a(4) =520a(5) =512a(6) =12288a(7) =32768a(8) =131584a(9) =266240a(10) =1310720a(11) =8388608a(12) =4325376a(13) =39059456a(14) =100663296