The 49ers dominated the first half and led 28-7 at halftime.  However, their second half offense stalled while the defense fell apart as the 49ers squandered a 21-point 3rd quarter lead and a 14-point lead in the final minutes of the 4th quarter.  The Chargers outscored the 49ers 31-7 after halftime.  Trailing 35-28, the Chargers drove down the field for the game-tying touchdown, converting two 4th downs on that drive.  The 49ers attempted to win in the final seconds of regulation, but Phil Dawson's 60 yard kick was way off.  The game went to overtime.  The 49ers got the ball first, but a fumble by Quinton Patton allowed the Chargers to have possession on their own 40.  The Chargers drove the 49ers' 22 yard line before kicking a field goal to hand the 49ers their fourth straight loss. The 49ers rushed for an astonishing 355 yards, including a 52-yard touchdown run by Frank Gore and a 90-yard touchdown run by Colin Kaepernick. The 49ers set an NFL record for most rushing yards in a loss The 49ers also scored 35 points, a season high.  However, their offensive struggles in the second half of games continued.  During the four game losing streak, the 49ers scored only 13 points in the second half of their games (outscored 65-13 by opponents in the second half of those games).  With the loss the 49ers fell to 7-8, ensuring that they would finish without a winning record for the first time since the 2010 season.
Question: How many points were the Chargers trailing by at halftime?
Answer: 21