What is the output when the program is executed?

public class TwoDArraySum {
    public static int sum(int[][] arr, int i, int j) {
        if (i == arr.length) return 0;
        if (j == arr[i].length) return sum(arr, i + 1, 0);
        return arr[i][j] + sum(arr, i, j + 1);
    }
    public static void main(String[] args) {
        int[][] matrix = { {1, 2, 3}, {4, 5, 6} };
        System.out.println(sum(matrix, 0, 0));
    }
}