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)
            if (min s t = '4') && (max s t = '7') then
              "4" + mymax ss ts
            else 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