Context: Following their road victory over the Steelers, the Broncos flew to McAfee Coliseum for an AFC West rematch with the Oakland Raiders.  In the first quarter, the Raiders score first with RB LaMont Jordan on a 1-yard TD run.  The Broncos would respond with QB Jake Plummer completing a 39-yard TD run with WR Javon Walker.  In the second quarter, Oakland kicker Sebastian Janikowski would get a 55-yard and a 20-yard field goal to give the Raiders a 13-7 halftime lead.  After a scoreless third quarter, Denver would score twice in the 4th Quarter with Plummer's 1-yard TD pass to FB Kyle Johnson and Jason Elam kicking a 24-yard field goal.  With the win, the Broncos improved to 7-2.

Question: How many points were the Raiders leading by at the half?

Answer:
6