Answer based on context:

The Colts began three straight home games in Week 8 with an interconference duel against the San Francisco 49ers.  Indianapolis would find themselves trailing in the first quarter as 49ers running back Frank Gore got a 64-yard touchdown run.  The Colts answered with kicker Matt Stover getting a 38-yard field goal.  In the second quarter, Indianapolis crept closer as Stover made a 33-yard field goal, yet San Francisco answered with quarterback Alex Smith completing an eight-yard touchdown pass to tight end Vernon Davis.  The Colts closed out the half with a 31-yard field goal from Stover. In the second half, Indianapolis narrowed San Francisco's lead to two as Stover booted a 40-yard field goal. In the fourth quarter, they executed a halfback option play as running back Joseph Addai's threw a 22-yard touchdown pass to wide receiver Reggie Wayne. Afterwards, the defense held off against the various comeback attempts from the 49ers. With the win, the Colts improved to 7-0 for the fourth time in five seasons and they set a franchise record with 15 consecutive regular season wins. Also, quarterback Peyton Manning (31/48 for 347 yards) joined Dan Marino, Brett Favre, and John Elway as the only players in NFL history to complete 4,000 career passes.  In addition, he became the fastest to reach the milestone as he reached it in 183 games.

How many yards was Stover's longest field goal of the first half?
38