Scanf.scanf "%s" (fun s ->
    let a = Array.make 13 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.(12) > 2 || fst a.(11) > 1 then print_endline "Impossible" else
    if fst a.(0) = 1 then (
        String.iter (fun c -> Printf.printf "%c\n" c) "abcdefghijklm"
    ) else
        Printf.printf "%c\n" "abcdefghijklm".[snd a.(0)]
)