Scanf.scanf "%s"(fun s ->
    let a = Array.make 26 0 in
    String.iter (fun c -> let q = int_of_char c - 97 in a.(q) <- a.(q) + 1) s;
    let a = Array.mapi (fun i v -> v, i) a in
    Array.sort compare a;
    if fst a.(25) = 2 then (
        Printf.printf "%c\n" @@ char_of_int (snd a.(19) + 97)
    ) else
        print_endline "Impossible"
)