import Data.Array
import Data.Array.ST
createArr::[Char]->Array Char Int
createArr xs = runSTArray $ do
                        ar <- newArray ('a','z') 0
                        mapM_  (writer ar) xs
                        return ar
                          where writer ar c = do
                                            v<-readArray ar c
                                            writeArray ar c (v+1)

solver::[Char]->String
solver xs = if (and.(map (\x->x <= 1)).elems.createArr) xs then "yes" else "no"

main::IO()
main= do
  d<-getLine
  putStr ((solver d)++"\n")