Nine stones are arranged in a straight line. They are counted from left to right as 1, 2, ..., 9, and then from right to left, so that the stone previously counted as 8 is counted as 10. The pattern is continued to the left until the stone previously counted as 1 is counted as 17. The pattern then reverses so that the stone originally counted as 2 is counted as 18, 3 as 19, and so on. The counting continues in this manner. Which of the original stones is counted as 999? Express your answer as a single digit which corresponds to the first digit assigned to that stone.
