fff ffi ffo ffs fft fif fii fis fit fof foi foo fos fot fsf fsi fso fss fst ftf fti fto fts ftt i-f iff ifi ifo ifs ift iif iii iio iis iit io- iof ioi ioo ios iot is- isf isi iso iss ist itf iti ito its itt o-i of- off ofi ofo ofs oft oif oii oio ois oit oo- oof ooi ooo oos oot os- osf osi oso oss ost ot- otf oti oto ots ott sff sfi sfo sfs sft sif sii sio sis sit sof soi soo sos sot ss- ssf ssi sso sss sst stf sti sto sts stt tff tfi tfo tfs tft tif tii tio tis tit to- tof toi too tos tot tsf tsi tso tss tst ttf tti tto tts ttt