/**
 * n + 1 > x * 1.08 >= n
 * (n + 1) / 1.08 > x >= n / 1.08
 */

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

  const x1 = n / 1.08;
  const x2 = (n + 1) / 1.08;

  const result = Math.ceil(x1);
  if (result <= Math.floor(x2)) {
    console.log(result);
  } else {
    console.log(":(");
  }
}

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