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


exception BadInput
fun findAns a b =
    let
        fun findAnsAux [] _ = raise BadInput
          | findAnsAux _ [] = raise BadInput
          | findAnsAux (#"4"::_) (#"7"::_) = a
          | findAnsAux (#"7"::_) (#"4"::_) = b
          | findAnsAux (h1::tl1) (h2::tl2) =
            if h1 > h2 then a
            else if h1 < h2 then b
            else findAnsAux tl1 tl2
    in
        if String.size a < String.size b then b
        else if String.size a > String.size b then a
        else findAnsAux (String.explode a) (String.explode b)
    end


val () =
    let
        val a = readStr ()
        val b = readStr ()
        val ans = findAns a b
    in
        print (ans ^ "\n")
    end