let g a b = Seq.zip a b |> Seq.tryFindIndex (fun (a,b) -> a<>b) |> Option.get
let f (s:string) (t:string) =
    if s.Length<>t.Length then
        s.Length>t.Length
    else
        let i = g s t
        match (s.[i],t.[i]) with
        | ('7','4') | ('4','7') -> not (s.[i]>t.[i])
        | _ -> s.[i]>t.[i]
    |> fun b -> if b then s else t

(stdin.ReadLine >> fun s -> s.Split ' ') () |> fun a -> f a.[0] a.[1] |> printfn "%s"