Troll Under a Bridge

There was a troll who lived under a bridge. The only way to get across to the other side was to take the bridge. It takes you one minute to cross the bridge even if you run. The troll sleeps for 30 seconds under the bridge. When you walk half way across the bridge, which would take you 30 seconds, the troll wakes up and pushes you back to where you started. How do you get across the bridge?