a-a a-b a-c aaa aab aac aak aao aas ab- aba abb abc abk abo abs ac- aca acb acc ack aco acs aka akb akc akk ako aks aoa aob aoc aok aoo aos as- asa asb asc ask aso ass baa bab bac bak bao bas bba bbb bbc bbk bbo bbs bca bcb bcc bck bco bcs bka bkb bkc bkk bko bks boa bob boc bok boo bos bsa bsb bsc bsk bso bss c-- caa cab cac cak cao cas cba cbb cbc cbk cbo cbs cca ccb ccc cck cco ccs cka ckb ckc cko cks co- coa cob coc cok coo cos csa csb csc csk cso css ka- kaa kab kac kak kao kas kba kbb kbc kbk kbo kbs kca kcb kcc kck kcs kkb kkc kkk kks koa kob koc kok koo kos ksa ksb ksc ksk kso kss oaa oab oac oak oao oas ob- oba obb obc obk obo obs oc- oca ocb occ ock oco ocs oka okb okc okk oko oks oo- ooa oob ooc ook ooo oos os- osa osb osc osk oso oss saa sab sac sak sao sas sba sbb sbc sbk sbo sbs sca scb scc sck sco scs sk- ska skb skc skk sko sks soa sob soc sok soo sos ss- ssa ssb ssc ssk sso sss