In week 8, the Lions flew to London, England to play in the NFL International Series against the Atlanta Falcons at Wembley Stadium. The game was played at 9:30 AM EDT, the first ever NFL game in that time slot. Like the previous game, the Lions were trailing by a large margin (down 21-0 at halftime) before mounting a huge comeback, scoring 22 unanswered points in the second half to win by a single point in the final minutes. The Falcons had touchdown catches of 7 and 1 yards respectively by Devonta Freeman and Bear Pascoe in the first quarter, then a 1-yard run by Steven Jackson in the second quarter. After halftime, a 22-yard Matt Prater field goal and a 59-yard TD pass from Matthew Stafford to Golden Tate narrowed the score to 21-10 by the end of the third quarter. Detroit got the ball on the Atlanta 8-yard line early in the fourth quarter after a Cassius Vaughn interception and return, settling for a 20-yard Prater field goal. A 5-yard touchdown reception by Theo Riddick made the score 21-19, but the Lions failed to convert a two-point conversion. The Falcons had the ball in Lions territory with under two minutes to play, but a holding penalty and a dropped pass both stopped the clock, forcing Atlanta to punt. With no time outs remaining and 1:38 on the clock, Detroit drove from its own 6-yard line to the Atlanta 25 with four seconds remaining in the game. Prater missed a 43-yard field goal attempt, but the whistle had blown before the snap because of a Detroit delay-of-game penalty. Prater then nailed a 48-yarder as time expired for the win. Calvin Johnson, still recovering from an ankle injury, missed a third straight game for the Lions, who also played without Reggie Bush and its top three tight ends (Eric Ebron, Brandon Pettigrew, and Joseph Fauria). Matthew Stafford's two touchdown passes gave him a team-record 120 in his career, breaking the tie he held with Bobby Layne at 118. The Falcons blew a halftime lead of 21 points or more for the first time in team history. The Lions entered their bye week atop the NFC North with a 6-2 record.
Question: How many points were the Falcons ahead at halftime?
Answer: 21