What's the most accurate description of what this program prints?

public class Main {
    static int counter = 0;

    public static void main(String[] args) throws InterruptedException {
        Runnable task = () -> {
            for (int i = 0; i < 10_000; i++) counter++;
        };
        Thread t1 = new Thread(task);
        Thread t2 = new Thread(task);
        t1.start(); t2.start();
        t1.join(); t2.join();
        System.out.println(counter);
    }
}