a-a a-b aaa aab aak aao aar aas ab- aba abb abk abo abr abs aka akb akk ako akr aks aoa aob aok aoo aor aos ar- ara arb ark aro arr ars as- asa asb ask aso asr ass baa bab bak bao bar bas bba bbb bbk bbo bbr bbs bka bkb bkk bko bkr bks boa bob bok boo bor bos bra brb brk bro brr brs bsa bsb bsk bso bsr bss ka- kaa kab kak kao kar kas kba kbb kbk kbo kbr kbs kkb kkk kkr kks koa kob kok koo kor kos kra krk kro krr krs ksa ksb ksk kso ksr kss oaa oab oak oao oar oas ob- oba obb obk obo obr obs oka okb okk oko okr oks oo- ooa oob ook ooo oor oos or- ora orb ork oro orr ors os- osa osb osk oso osr oss ra- raa rab rak rao rar ras rba rbb rbk rbo rbr rbs rkb rkk rko rks roa rob rok roo ror ros rra rrb rrk rrr rrs rsa rsb rsk rso rsr rss saa sab sak sao sar sas sba sbb sbk sbo sbr sbs sk- ska skb skk sko skr sks soa sob sok soo sor sos sr- sra srb srk sro srr srs ss- ssa ssb ssk sso ssr sss