b-u bbb bbk bbr bbs bbu bkb bkk bkr bks brb brk brr brs bru bsb bsk bsr bss bsu bub buk bur bus buu kbb kbk kbr kbs kbu kkb kkk kkr kks kku krk krr krs kru ksb ksk ksr kss ksu kub kuk kur kus kuu rbb rbk rbr rbs rbu rkb rkk rks rku rrb rrk rrr rrs rru rsb rsk rsr rss rsu rub ruk rur rus ruu sbb sbk sbr sbs sbu sk- skb skk skr sks sku sr- srb srk srr srs ss- ssb ssk ssr sss ssu sub suk sur sus suu ubb ubk ubr ubs ubu ukb ukk ukr uku ur- urb urk urr urs uru usb usk usr uss usu uur uus uuu