context: After a turnover, the Steelers scored the first points on a Jeff Reed 37-yard field goal.  Brian Westbrook left the game with an ankle injury early in the second quarter, but Correll Buckhalter finished the drive with a 20-yard touchdown catch.  The touchdown was the 176th for Donovan McNabb, surpassing Ron Jaworski on the franchise list.  The Philadelphia defense began to take over at this point, sacking Roethlisberger six times in the quarter.  A fumble recovery by Brodrick Bunkley after a sack led to a David Akers field goal.  Pittsburgh would end the half with Reed kicking a 53-yard field goal to make it 10-6.  Without Westbrook, the Eagles' offense struggled to score, but the defense continued to shut down the Steelers.  An intentional grounding penalty on Roethlisberger from his own end zone resulted in a safety.  A diving sack and fumble recovery by Brian Dawkins led to a field goal and sealed the win, giving the Eagles a 15-6 lead.  The defense sacked Roethlisberger and Byron Leftwich nine times, forced two fumbles, and intercepted a pass.  Punter Sav Rocca also contributed to the second-half shutout with his punts.  The Eagles improved to 2-1 with the win.
question: How many points was Pittsburg trailing by at the half?
answer: 4