import java.util.Random;

interface OnEndCallback {
    public void onEnd(final int output);
}

class R implements Runnable {
    private final OnEndCallback cb;
    private final int input;

    public R(final int input, final OnEndCallback cb) {
    this.input = input;
    this.cb = cb;
    }

    public void run() {
    System.out.println("" + Thread.currentThread() + ": input=" + input);
    try {
        Thread.sleep((new Random()).nextInt(3000));
    } catch (InterruptedException unused) {
    }
    cb.onEnd(input + 1);
    }
}

class Sync implements OnEndCallback {
    public static void main(final String[] args) {
    final Sync s = new Sync();
    s.go(0);
    }

    void go(final int input) {
    if (input < 5) {
        new Thread(new R(input, this)).start();
    }
    }

    public void onEnd(final int output) {
    go(output);
    }
}