fun readStr () =
    let
        fun scan reader stream = SOME (StringCvt.splitl (not o Char.isSpace) reader (StringCvt.skipWS reader stream))
    in
        valOf (TextIO.scanStream scan TextIO.stdIn)
    end


fun findAns s =
    let
        fun findAnsAux nil _ = "YES"
          | findAnsAux (h :: tl) used = 
            if Array.sub (used, Char.ord h) then "NO"
            else (Array.update (used, Char.ord h, true); findAnsAux tl used)
    in
        findAnsAux (String.explode s) (Array.array (Char.ord #"Z" + 1, false))
    end



val () =
    let
        val s = readStr ()

        val ans = findAns s
    in
        print (ans ^ "\n")  
    end