a-a a-b aaa aab aae aaf aak aao ab- aba abb abe abf abk abo aea aeb aee aef aek aeo af- afa afb afe aff afk afo aka akb ake akk ako aoa aob aoe aof aok aoo baa bab bae baf bak bao bba bbb bbe bbf bbk bbo be- bea beb bee bef bek beo bfa bfb bfe bff bfo bka bkb bke bkf bkk bko boa bob boe bof bok boo eaa eab eae eaf eak eao eba ebb ebe ebf ebk ebo eea eeb eee eef eek eeo ef- efa efb efe eff efo eka ekb eke ekf eko eo- eoa eob eoe eof eok eoo faa fab fae faf fak fao fba fbb fbe fbf fbk fbo fea feb fee fef fek feo ffa ffe fff ffk ffo fka fkb fkf fkk foa fob foe fof fok foo ka- kaa kab kae kaf kak kao kba kbb kbe kbf kbk kbo ke- kea keb kee kef kek keo kfa kff kfo kkb kke kkk koa kob koe kof kok koo oaa oab oae oaf oak oao ob- oba obb obe obf obk obo oea oeb oee oef oeo of- ofa ofb ofe off ofo oka okb oke okk oko oo- ooa oob oof ook ooo