a-ak a-ok aaaa aaab aaac aaaf aaba aabb aabc aaca aacb aacc aacf aack aaco aafa aafb aafc aaff aaka aaoo abaa abab abac abaf abak abb- abba abbb abbo abca abcb abcc abcf abff aboa abob abof abok aboo acaa acab acac acba acca accc accf acco acfc acoa afaa afak afao afca afcb afcc affa affc affo afka afoa afoo akaa akao akca akka akko akok aoba aoca baaa baaf baak baba babb babo bac- baca bacc back baco bafc baff bafo bak- baka bako bbbb bbfc bcaa bcbc bcca bccc bcfc bcof bfca boab boac boak boba bobb bobo boca bock boco bofa boff boka boko boob booc boof book c-bo caaa caab caac caaf caba cabf cabo cac- caca cacc cack caco cafa cafb cafc caff caka cbac cbbc cbca cbcc cbfb cbfc cbka cbko ccaa ccab ccac ccaf ccba ccbc ccca cccb cccc cccf ccfa ccfc ccff ccob ccoc cfaa cfab cfac cfao cfbc cfca cfcb cfcf cfco cffb cfoa ckac ckbc ckck ckco ckoa ckob ckoc ckoo coak coba cobb cobo coca cock coco cofc coff cok- coka coko coob coof cook f--k faaa faac faba fabo faca facc fack fafa fafc faff fafo fako fcab fcba fcca fccf ffcc ffff fkab foaf foak foba foca fock foco fofo foof fook kaaa kaab kaaf kaak kaba kabb kabc kabf kabo kacb kacc kaco kafa kafc kaff kaka kakc kakk kako kaoa kaoc kaok kbaa kbab kbac kbak kbba kbbb kbbc kbbf kbbk kbbo kbca kbcb kbck kbco kbfb kbfc kbff kbfk kbfo kbkb kbkf kbkk kbko kboa kbob kboc kbok kboo kcaa kcab kcac kcba kcbc kcbf kcbk kcca kccb kccc kcck kcfa kcfb kcfc kcfo kckc kckf kckk kcko kcob kcof kfab kfac kfbc kfbk kfcf kfco kffa kffb kfff kffk kfka kfkb kfok kfoo kkaa kkac kkba kkbb kkbo kkcb kkck kkco kkkk kkoa kkob kkoo koab koac koak koba kobb kobc kobk kobo koca kocb kocf kock kofa kofc koff kofk kofo koka kokb kokc kokf kokk koko koob kooc kook kooo oafc oafo obak obba obbo obkb obok ocab ocac ocak ocbc occa occc ocfa off- offa offc offo okak okka okko okok ooaa ooak ooba ooff oooo