"use strict";

function Main(input) {
  let x = Math.ceil(input/1.08);
  let x2 = Math.floor(x*1.08);
  if(input == x2){
    console.log(Math.ceil(x));
  } else {
    console.log(":(");
  }
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
