a-a a-b aaa aab aao aar aas ab- aba abb abo abr abs aoa aob aoo aor aos ar- ara arb aro arr ars as- asa asb aso asr ass baa bab bao bar bas bba bbb bbo bbr bbs boa bob boo bor bos bra brb bro brr brs bsa bsb bso bsr bss oaa oab oao oar oas ob- oba obb obo obr obs oo- ooa oob ooo oor oos or- ora orb oro orr ors os- osa osb oso osr oss ra- raa rab rao rar ras rba rbb rbo rbr rbs roa rob roo ror ros rra rrb rrr rrs rsa rsb rso rsr rss saa sab sao sar sas sba sbb sbo sbr sbs soa sob soo sor sos sr- sra srb sro srr srs ss- ssa ssb sso ssr sss