a-a a-b a-i aaa aab aai aao aar aas ab- aba abb abi abo abr abs ai- aia aib aii aio air ais aoa aob aoi aoo aor aos ar- ara arb ari aro arr ars as- asa asb asi aso asr ass baa bab bai bao bar bas bba bbb bbi bbo bbr bbs bi- bia bib bii bio bir bis boa bob boi boo bor bos bra brb bri bro brr brs bsa bsb bsi bso bsr bss ia- iaa iab iai iao iar ias iba ibb ibi ibo ibr ibs iia iib iii iio iir iis io- ioa iob ioi ioo ior ios ir- ira irb iri iro irr irs is- isa isb isi iso isr iss o-i oaa oab oai oao oar oas ob- oba obb obi obo obr obs oia oib oii oio oir ois oo- ooa oob ooi ooo oor oos or- ora orb ori oro orr ors os- osa osb osi oso osr oss ra- raa rab rai rao rar ras rba rbb rbi rbo rbr rbs ria rib rii rio rir ris roa rob roi roo ror ros rra rrb rri rrr rrs rsa rsb rsi rso rsr rss saa sab sai sao sar sas sba sbb sbi sbo sbr sbs sia sib sii sio sir sis soa sob soi soo sor sos sr- sra srb sri sro srr srs ss- ssa ssb ssi sso ssr sss