open Printf
open Scanf

let tax x = x * 108 / 100
  
let solve n =
  let rec f s l =
    let m = (s + l) / 2 in
    let t = tax m in
    if s + 1 = l then ":("
    else if t = n then string_of_int m
    else if t > n then f s m
    else f m l
  in f 0 50000

let () =
  scanf "%d " solve |> printf "%s\n"
