a-ak aaaa aaab aaac aaaf aaba aabb aabc aaca aacb aacc aacf aack aafa aafb aafc aaff aaka abaa abab abac abaf abak abb- abba abbb abca abcb abcc abcf abff acaa acab acac acba acca accc accf acfc afaa afak afca afcb afcc affa affc afka akaa akca akka baaa baaf baak baba babb bac- baca bacc back bafc baff bak- baka bbbb bbfc bcaa bcbc bcca bccc bcfc bfca caaa caab caac caaf caba cabf cac- caca cacc cack cafa cafb cafc caff caka cbac cbbc cbca cbcc cbfb cbfc cbka ccaa ccab ccac ccaf ccba ccbc ccca cccb cccc cccf ccfa ccfc ccff cfaa cfab cfac cfbc cfca cfcb cfcf cffb ckac ckbc ckck f--k faaa faac faba faca facc fack fafa fafc faff fcab fcba fcca fccf ffcc ffff fkab kaaa kaab kaaf kaak kaba kabb kabc kabf kacb kacc kafa kafc kaff kaka kakc kakk kbaa kbab kbac kbak kbba kbbb kbbc kbbf kbbk kbca kbcb kbck kbfb kbfc kbff kbfk kbkb kbkf kbkk kcaa kcab kcac kcba kcbc kcbf kcbk kcca kccb kccc kcck kcfa kcfb kcfc kckc kckf kckk kfab kfac kfbc kfbk kfcf kffa kffb kfff kffk kfka kfkb kkaa kkac kkba kkbb kkcb kkck kkkk