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

  var lowest = Math.floor(x);
  if (
    Math.floor(lowest * 1.08) !== Math.floor(input) &&
    Math.floor((lowest + 1) * 1.08 !== Math.floor(input))
  ) {
    console.log(':(');
  } else {
    console.log(x);
  }
}
Main(require('fs').readFileSync('/dev/stdin', 'utf8'));