a-a a-b a-c aaa aab aac aak aas ab- aba abb abc abk abs ac- aca acb acc ack acs aka akb akc akk aks as- asa asb asc ask ass baa bab bac bak bas bba bbb bbc bbk bbs bca bcb bcc bck bcs bka bkb bkc bkk bks bsa bsb bsc bsk bss c-- caa cab cac cak cas cba cbb cbc cbk cbs cca ccb ccc cck ccs cka ckb ckc cks csa csb csc csk css ka- kaa kab kac kak kas kba kbb kbc kbk kbs kca kcb kcc kck kcs kkb kkc kkk kks ksa ksb ksc ksk kss saa sab sac sak sas sba sbb sbc sbk sbs sca scb scc sck scs sk- ska skb skc skk sks ss- ssa ssb ssc ssk sss