Int 13, Combat Expertise.
You do not provoke an attack of opportunity when you attempt to trip an opponent while you are unarmed. You also gain a +4 bonus on your Strength check to trip your opponent.
If you trip an opponent in melee combat, you immediately get a melee attack against that opponent as if you hadn’t used your attack for the trip attempt.
Without this feat, you provoke an attack of opportunity when you attempt to trip an opponent while you are unarmed.
At 6th level, a monk may select Improved Trip as a bonus feat, even if she does not have the prerequisites.
A fighter may select Improved Trip as one of his fighter bonus feats.