a-ak aaaa aaab aaac aaba aabb aabc aaca aacb aacc aack aaka abaa abab abac abak abb- abba abbb abca abcb abcc acaa acab acac acba acca accc akaa akca akka baaa baak baba babb bac- baca bacc back bak- baka bbbb bcaa bcbc bcca bccc caaa caab caac caba cac- caca cacc cack caka cbac cbbc cbca cbcc cbka ccaa ccab ccac ccba ccbc ccca cccb cccc ckac ckbc ckck kaaa kaab kaak kaba kabb kabc kacb kacc kaka kakc kakk kbaa kbab kbac kbak kbba kbbb kbbc kbbk kbca kbcb kbck kbkb kbkk kcaa kcab kcac kcba kcbc kcbk kcca kccb kccc kcck kckc kckk kkaa kkac kkba kkbb kkcb kkck kkkk