After a tough win over the Texans, the Steelers stayed home for a game against the Colts.  In the first quarter, the Colts scored first as Adam Vinatieri nailed a 31-yard field goal for a 3-0 lead.  However, the Steelers took the lead later on in the quarter when Ben Roethlisberger found Markus Wheaton on an 18-yard TD pass for a 7-3 game.  In the 2nd quarter, the Steelers would score again increasing their lead as Roethlisberger found Martavis Bryant on a 5-yard TD pass for a 14-3 game.  This was followed by William Gay picking off Andrew Luck and returning it 33 yards for a TD as they increased their lead to 21-3.  The Colts however responded as Andrew Luck threw a 21-yard TD pass to Dwayne Allen for a 21-10 game.  However, the Steelers pulled away as Roethlisberger would find Antonio Brown on 2 consecutive TD passes:  from 8 and 47 yards out to increase their lead at first from 28-10 to 35-10.  The Colts responded with 10 straight points first being Luck who found T. Y. Hilton on a 28-yard TD Pass for a 35-17 game followed by Vinatieri kicking a 23-yard field goal before halftime to make the score 35-20.  The Steelers went back to work in the 3rd quarter as Roethlisberger and Bryant hooked up again on a 2-yard TD pass to increase their lead making the score 42-20.  The Colts however were able to come within 8 later on when Ahmad Bradshaw ran for a 12-yard TD followed by Luck finding Donte Moncrief on a 31-yard TD pass to make the score 42-27 and then 42-34.  In the 4th quarter, it was all Steelers when a penalty was enforced on Luck in the end zone for a safety making the score 44-34.  The Steelers were able to seal the game later on when Roethlisberger found Heath Miller on an 11-yard TD pass for a final score of 51-34. Roethlisberger set franchise records with 522 yards passing and six touchdowns while picking up his 100th victory in his 150th start. Roethlisberger's yardage total was the fourth highest in NFL history. He became the first player in NFL history to go over 500 yards passing twice as the Steelers won consecutive games for the first time this season.
Question: How many points did Pittsburgh lead by at halftime?
Answer: 15