public class Semaphore {
  byte value = 1;
  
  public Semaphore(byte init_value) {
    value = init_value;
  }
  
  public synchronized void down() {
    while(value==0) {
      try {
        wait();
      } catch(InterruptedException ex) {}
    }
    value--;
  }
  
  public synchronized void up() {
    value++;
    notifyAll();
  }
}