public class SemAlt {

    static Semaphore c = new Semaphore(1);
    static Semaphore r = new Semaphore(0);

    static class Caller extends Thread {

	public void run () {
	    int i;
	    for (i = 0; i < 10; i++) {
		c.P();
		System.out.println("Call");
		r.V();
	    }
	}
    }

    static class Returner extends Thread {

	public void run () {
	    int i;
	    for (i = 0; i < 10; i++) {
		r.P();
		System.out.println("Return");
		c.V();
	    }
	}
    }    

    public static void main(String[] args) {
	Caller c = new Caller();
	Returner r = new Returner();
	c.start();
	r.start();
    }
}