Array Indexing with Pre-Increment

Consider the following code segment. What is printed when it is executed?

public class ComplexIndex {
    public static void main(String[] args) {
        int[] values = {3, 6, 9, 12};
        int index = 1;
        values[index] = values[index] + values[++index];
        System.out.println(values[1]);
    }
}