function Main(input) {
    const n = input.split("\n")[0];
    num = n / 1.08
    var integer = Math.floor(num);
    var decimal = parseFloat("0."+(String(num)).split(".")[1]);

    const mod = n % 108
    var period1 = Math.floor(decimal * 10)
    if (mod === 0) {
        console.log(n / 1.08);
    } else if (period1 === 0) {
        console.log(":(");
    } else {
        console.log(Math.floor(n / 1.08) + 1);
    }
  
    }
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
  