let after = stdin.ReadLine() |> int

let before1 = 
  after
  |> float 
  |> fun x -> x / 1.08
  |> int
let before2 = before1 + 1
if int (float before1 * 1.08) = after then
  printfn "%d" before1
else if int (float before2 * 1.08) = after then
  printfn "%d" before2
else 
  printfn ":("