Let $S = \{(x, y) \in \mathbb{Z} \times \mathbb{Z} : 1 \le x \le 3, 1 \le y \le 3\}$ be a sample space where each of the 9 points is equally likely to be selected. If a point $(x, y)$ is chosen at random from $S$, what is the probability that the sum $x + y$ is a prime number?