a-a a-b aaa aab aak aan aao ab- aba abb abk abn abo aka akb akk akn ako an- ana anb ank ann ano aoa aob aok aon aoo baa bab bak ban bao bba bbb bbk bbn bbo bka bkb bkk bkn bko bna bnb bnk bnn bno boa bob bok bon boo ka- kaa kab kak kan kao kba kbb kbk kbn kbo kkb kkk kkn kn- kna knk knn kno koa kob kok kon koo naa nab nak nan nao nba nbb nbk nbn nbo nka nkk nkn nko nna nnb nnk nnn nno no- noa nob nok non noo oaa oab oak oan oao ob- oba obb obk obn obo oka okb okk oko on- ona onb onk onn ono oo- ooa oob ook oon ooo