context:  After winning over the Chiefs on the road, the Falcons returned home to face a newly formed Peyton Manning-led Broncos team. The Falcons got off to a really fast start scoring 20 unanswered points in the first half thanks to four Broncos turnovers. Three plays into the game, Peyton Manning on 3rd-and-12 from his own 12, was intercepted by William Moore at the 34. He returned it 33 yards to the 1 yard line. Three plays later, Michael Turner's 1-yard touchdown run gave the Falcons a 7-0 lead Another interception by Manning led to Matt Bryant's 37-yard field goal to take a 10-0 lead. In the 2nd quarter, the team moved up 13-0 after a Willis McGahee fumble which led to Bryant's 42-yard field goal then Ryan's 1-yard touchdown pass to Tony Gonzalez for a 20-0 lead. The Broncos scored before halftime with Peyton Manning's 17-yard touchdown pass to Demaryius Thomas to cut the lead to 20-7 at halftime. In the third quarter, the Falcons moved ahead by 20 again with Matt Ryan's 4-yard touchdown pass Roddy White for a 27-7 lead. The Broncos then tried to rally a comeback in the 4th quarter with Willis McGahee running for 2-yard touchdowns to cut the lead to 27-21, but the Falcons were able to hold the Broncos off and win the game.
question: How many points were the Broncos trailing by at halftime?
answer: 13