Given the interface and classes below, what will be the output?

Given the interface and classes below, what will be the output?

interface Levels {
  const LOW = 1;
  const HIGH = 10;
}
class Base implements Levels {
  public function getLow() {
    return self::LOW;
  }
}
class Advanced extends Base {
  public function getHigh() {
    return Levels::HIGH;
  }
}
$adv = new Advanced();
echo $adv->getLow() + $adv->getHigh();