ˈaɪ.ˈiː, ɪf ˈɔːl ðɪ ˈʌðɚz ˈækt tɹˈuːθfəli, ðˈɛn ɪɾ ɪz ˈɔːlsoʊ bˈɛst ɔːɹ æt lˈiːst nˌɑːt wˈɜːs fɔːɹ juː təbi tɹˈuːθfəl.
I.e, if all the others act truthfully, then it is also best or at least not worse for you to be truthful.