The Redskins began their season at home on Sunday night against their NFC East rival, the Dallas Cowboys.  In the first quarter, Washington delivered the opening punch as kicker Graham Gano got a 29-yard field goal.  The Redskins would then close out the second quarter with cornerback DeAngelo Hall returning a fumble 25 yards for a touchdown. The Cowboys would get on the board in the third quarter as quarterback Tony Romo found wide receiver Miles Austin on a 4-yard touchdown pass.  Washington would respond in the fourth quarter as Gano nailed a 49-yard field goal.  Afterwards, Dallas appeared to have won the game as Romo completed a 13-yard touchdown pass to wide receiver Roy Williams, but a holding call on offensive tackle Alex Barron preserved the Redskins victory.
Question: how many yards did romo complete to williams?
Answer: 13