There is a single-plank bridge. Only one person can cross the bridge without any stop. The pedestrian can cross the bridge from west to east or from east to west. If there is more than 0 pedestrian crossing the bridge from west to east, no pedestrian can try to cross the bridge from east to west, but more than 1 pedestrian can cross the bridge from west to east, and vice versa. The pedestrian can cross the bridge immediately if there is no pedestrian on the bridge. Use Semaphore to solve this problem.