function Main(input) {
  var ans = Math.floor(input);

  var x = ans / 1.08;

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

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