(*
ocamlfind ocamlopt -package batteries -linkpkg main.ml -o a.out
*)
open Batteries

let () =
        Scanf.sscanf (read_line ()) "%f" (
        fun n -> 
                let a = n /. 1.08 in
                let fa = truncate ((floor a) *. 1.08) in
                let ca = truncate ((ceil a) *. 1.08) in
                        if fa = int_of_float n then
                                string_of_int (truncate (floor a))
                        else if ca = int_of_float n then
                                string_of_int (truncate (ceil a))
                        else
                                ":("
        ) |> Printf.printf "%s\n"
