-er a-a a-b a-c aaa aab aac aae aak aao aar aas ab- aba abb abc abe abk abo abr abs ac- aca acb acc ace ack aco acr acs aea aeb aec aee aek aeo aer aes aka akb akc ake akk ako akr aks aoa aob aoc aoe aok aoo aor aos ar- ara arb arc are ark aro arr ars as- asa asb asc ase ask aso asr ass baa bab bac bae bak bao bar bas bba bbb bbc bbe bbk bbo bbr bbs bca bcb bcc bce bck bco bcr bcs be- bea beb bec bee bek beo ber bes bka bkb bkc bke bkk bko bkr bks boa bob boc boe bok boo bor bos bra brb bre brk bro brr brs bsa bsb bsc bse bsk bso bsr bss c-- caa cab cac cae cak cao car cas cba cbb cbc cbe cbk cbo cbr cbs cca ccb ccc cce cck cco ccr ccs ce- cea ceb cec cee cek ceo cer ces cka ckb ckc cko cks co- coa cob coc coe cok coo cor cos cra crb crc cre crk cro crr crs csa csb csc cse csk cso csr css eaa eab eac eae eak eao ear eas eba ebb ebc ebe ebk ebo ebr ebs ec- eca ecb ecc ece eck eco ecr ecs eea eeb eec eee eek eeo eer ees eka ekb ekc eke eko ekr eks eo- eoa eob eoc eoe eok eoo eor eos era erb erc ere erk ero err ers es- esa esb esc ese esk eso esr ess ka- kaa kab kac kae kak kao kar kas kba kbb kbc kbe kbk kbo kbr kbs kca kcb kcc kce kck kcr kcs ke- kea keb kec kee kek keo ker kes kkb kkc kke kkk kkr kks koa kob koc koe kok koo kor kos kra krc krk kro krr krs ksa ksb ksc kse ksk kso ksr kss oaa oab oac oae oak oao oar oas ob- oba obb obc obe obk obo obr obs oc- oca ocb occ ock oco ocr ocs oea oeb oec oee oeo oer oes oka okb okc oke okk oko okr oks oo- ooa oob ooc ook ooo oor oos or- ora orb orc ore ork oro orr ors os- osa osb osc ose osk oso osr oss ra- raa rab rac rae rak rao rar ras rba rbb rbc rbe rbk rbo rbr rbs rca rcb rcc rce rco rcr rcs re- rea reb rec ree rek reo rer res rkb rkc rke rkk rko rks roa rob roc roe rok roo ror ros rra rrb rrc rrk rrr rrs rsa rsb rsc rse rsk rso rsr rss saa sab sac sae sak sao sar sas sba sbb sbc sbe sbk sbo sbr sbs sca scb scc sce sck sco scr scs se- sea seb sec see sek seo ser ses sk- ska skb skc ske skk sko skr sks soa sob soc soe sok soo sor sos sr- sra srb src sre srk sro srr srs ss- ssa ssb ssc sse ssk sso ssr sss