fun quicksort [] = []
  | quicksort (h::tl) =
    let 
        val (s, b) =
            List.foldl
                (fn (x, (small, big)) =>
                    if x <= h then (x::small, big)
                    else (small, x::big))
                ([], [])
                tl
    in
        (quicksort s) @ [h] @ (quicksort b)
    end


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 replaceFromLast toRemove toAdd l =
    let
        fun replace [] = []
          | replace (h::tl) = 
            if h = toRemove then toAdd :: tl
            else replace tl
    in
        List.rev (replace (List.rev l))
    end


fun findAns s =
    let
        val asInt = List.map (fn ch => Char.ord ch) (String.explode s)
        val sorted = List.rev (quicksort asInt)
       
        fun findAnsAux [] _ = []
          | findAnsAux _ [] = []
          | findAnsAux (oh::ot) (dh::dt) = 
            if oh <> dh then dh :: replaceFromLast dh oh ot
            else oh :: findAnsAux ot dt
    in
        String.implode (List.map (fn i => Char.chr i) (findAnsAux asInt sorted))
    end


val () =
    let
        val n = readStr ()

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