open System
open System.Collections.Generic

[<EntryPoint>]
let main argv =
  let [| a; b |] = Console.ReadLine().Split(' ')

  let result =
    if a.Length <> b.Length then
      if a.Length > b.Length
      then a
      else b
    else
      let rec mymax (x: string) (y: string) =
          if x.Length = 0
          then ""
          else
            let s, ss = x.[0], x.Substring(1)
            let t, ts = y.[0], y.Substring(1)
            match (s, t) with
            | ('4', '7') -> x
            | ('7', '4') -> y
            | _ ->
                if s <> t
                then if s > t then x else y
                else (string s) + mymax ss ts
      mymax a b

  printfn "%s" result

  //exit code
  0