function Main(input) {
  var x = input / 1.08;

  var lowest = Math.floor(x);
  var highest = Math.ceil(x);

  if (
    Math.floor(
      Math.floor(lowest * 1.08) === Math.floor(input) &&
        Math.floor(highest * 1.08 === Math.floor(input))
    )
  ) {
    console.log(lowest);
  } else if (
    Math.floor(lowest * 1.08) < Math.floor(input) &&
    Math.floor(highest * 1.08 > Math.floor(input))
  ) {
    console.log(':(');
  } else {
    console.log(lowest);
  }
}
Main(require('fs').readFileSync('/dev/stdin', 'utf8'));