a-a a-b aaa aab aak aao aar ab- aba abb abk abo abr aka akb akk ako akr aoa aob aok aoo aor ar- ara arb ark aro arr baa bab bak bao bar bba bbb bbk bbo bbr bka bkb bkk bko bkr boa bob bok boo bor bra brb brk bro brr ka- kaa kab kak kao kar kba kbb kbk kbo kbr kkb kkk kkr koa kob kok koo kor kra krk kro krr oaa oab oak oao oar ob- oba obb obk obo obr oka okb okk oko okr oo- ooa oob ook ooo oor or- ora orb ork oro orr ra- raa rab rak rao rar rba rbb rbk rbo rbr rkb rkk rko roa rob rok roo ror rra rrb rrk rrr