public class Sema { 
    private int x;
    public Sema(int n) { x = n; }
    public synchronized void up() {
        x++; if (x <= 0) this.notify();
    }
    public synchronized void down() 
                throws InterruptedException {
        x--; while (x < 0) this.wait();
    }
} // end of class Sema