function Main(inputs) {
  const input = inputs.split("\n");
  const n = Number(input[0].split(" ")[0]);

  const result = Array(50001)
    .fill(0)
    .map((_, i) => (Math.floor(i * 1.08)))
    .findIndex((i) => i === n);
  console.log(result === -1 ? ":(" : result);
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));
