let () =
    let main () =
        let n = int_of_string (read_line ()) in
        let f1 = floor (float n /. 1.08) in
        let f2 = ceil  (float n /. 1.08) in
        let b1 = int_of_float (floor (f1 *. 1.08)) in
        let b2 = int_of_float (floor (f2 *. 1.08)) in
        if b1 = n then Printf.printf "%d\n" (int_of_float f1) else
        if b2 = n then Printf.printf "%d\n" (int_of_float f2) else print_endline ":("
    in
    main ()