open System

let solve (s : string) =
    let pairs =
        Seq.groupBy id s
        |> Seq.map (fun (c, cSeq) -> (c, Seq.length cSeq))
        |> Seq.toList
    let maxPair = List.maxBy (fun (_, cnt) -> cnt) pairs |> fun (c, cnt) -> cnt
    if maxPair > 2 then
        "Impossible"
    else
        let single =
            List.filter (fun (_, cnt) -> cnt = 1) pairs
            |> List.map (fun (c, _) -> c)
        if List.length single > 1 then
            "Impossible"
        else
            new String(single.[0], 1)

let () =
    let s = stdin.ReadLine()
    solve s
    |> printfn "%s"