public class Semaphore {
    int x;

    public Semaphore (int x) { this.x = x; }

    public synchronized void P () {
        while (x == 0)
            try { wait(); }
            catch (java.lang.InterruptedException e) { };
        x = x - 1;
    }

    public synchronized void V () {
        x = x + 1;
        notify();
    }
}