let (|Possible|Impossible|) (tup : (char * int)[]) =
    let getN n = tup |> Array.where(fun (_, b) -> b = n)
    
    let num    = tup |> Array.length
    let pair   = getN 2
    let lonly  = getN 1
    let pairNum  = pair  |> Array.length     
    let lonlyNum = lonly |> Array.length
    
    if pairNum + lonlyNum = num && lonlyNum = 1 then
        let key,_ = lonly |> Array.head        
        Possible key
    else
        Impossible
    
let S = stdin.ReadLine()

S.ToCharArray() |> Array.countBy(fun x -> x)
|> function Possible x -> string x | Impossible -> "Impossible"
|> printfn "%s"