a-a a-b a-c aaa aab aac aak aao aay ab- aba abb abc abk abo aby ac- aca acb acc ack aco acy aka akb akc akk ako aoa aob aoc aok aoo ay- aya ayc ayk ayo baa bab bac bak bao bay bba bbb bbc bbk bbo bby bca bcb bcc bck bco bcy bka bkb bkc bkk bko boa bob boc bok boo boy by- bya byc byk byo c-- caa cab cac cak cao cay cba cbb cbc cbk cbo cby cca ccb ccc cck cco cka ckb ckc cko cky co- coa cob coc cok coo coy cya cyb cyc cyk cyo ka- kaa kab kac kak kao kay kba kbb kbc kbk kbo kca kcb kcc kck kkb kkc kkk kky koa kob koc kok koo koy kya kyc kyo oaa oab oac oak oao ob- oba obb obc obk obo oby oc- oca ocb occ ock oco oka okb okc okk oko oo- ooa oob ooc ook ooo oya oyo yaa yab yac yak yao yay ybc yby yca ycc yco ycy yka ykk yob yoc yok yoo yoy yyb yyc yyy