a-a a-b aaa aab aak aao aas ab- aba abb abk abo abs aka akb akk ako aks aoa aob aok aoo aos as- asa asb ask aso ass baa bab bak bao bas bba bbb bbk bbo bbs bka bkb bkk bko bks boa bob bok boo bos bsa bsb bsk bso bss ka- kaa kab kak kao kas kba kbb kbk kbo kbs kkb kkk kks koa kob kok koo kos ksa ksb ksk kso kss oaa oab oak oao oas ob- oba obb obk obo obs oka okb okk oko oks oo- ooa oob ook ooo oos os- osa osb osk oso oss saa sab sak sao sas sba sbb sbk sbo sbs sk- ska skb skk sko sks soa sob sok soo sos ss- ssa ssb ssk sso sss