public class Lesson2 {
    private static int i;

    public static void main(String[] args) {
        Thread t1 = new Thread(new Thread2());
        Thread t2 = new Thread(new Thread2());
        t1.start();
        t2.start();
        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(i);
    }

    static void incr() {
        i++;
    }
}

class Thread2 implements Runnable {
    @Override
    public void run() {
        synchronized ( ЧТО ТУТ ПИСАТЬ??? ) {
            for(int i=0; i<2000; i++) {
                Lesson2.incr();
            }
        }
    }
}