Synchronizing Compound ArrayList Operations

Consider the provided code snippet. Why is it necessary to enclose the sequence of get, remove, and add operations within a single synchronized block?

import java.util.ArrayList;

public class CompoundOperation {
    static ArrayList<Integer> list = new ArrayList<>();
    public static void main(String[] args) {
        // Pre-populate the list
        for (int i = 0; i < 10; i++) {
            list.add(i);
        }
        Thread t1 = new Thread(() -> {
            synchronized(list) {
                int val = list.get(0);
                list.remove(0);
                list.add(val + 100);
            }
        });
        Thread t2 = new Thread(() -> {
            synchronized(list) {
                int val = list.get(1);
                list.remove(1);
                list.add(val + 200);
            }
        });
        t1.start();
        t2.start();
    }
}