a-a a-b aaa aab aan aao aar ab- aba abb abn abo abr an- ana anb ann ano anr aoa aob aon aoo aor ar- ara arb arn aro arr baa bab ban bao bar bba bbb bbn bbo bbr bna bnb bnn bno bnr boa bob bon boo bor bra brb brn bro brr naa nab nan nao nar nba nbb nbn nbo nbr nna nnb nnn nno no- noa nob non noo nor nra nrb nrn nro nrr oaa oab oan oao oar ob- oba obb obn obo obr on- ona onb onn ono onr oo- ooa oob oon ooo oor or- ora orb orn oro orr ra- raa rab ran rao rar rba rbb rbn rbo rbr rna rnb rnn rno rnr roa rob ron roo ror rra rrb rrn rrr