let () =
  let n = read_int () in
  let ans = ref (-1) in
  for i = 0 to n do
    let x = float_of_int i *. 1.08 |> int_of_float in
    if x = n then ans := i else ()
  done;
  if !ans >= 0 then print_int !ans else print_string ":(";
  print_newline ()
