<image> In 2016, Nanjing launched the event "Low-Carbon Travel for All, Creating a Green Nanjing." By downloading the mobile app "My Nanjing," green travel will earn points, which can be exchanged for cards according to the rules shown in the image. A citizen has no more than 650 points and has exchanged a total of 6 "Leaf" and "Tree" cards. What is the maximum number of "Tree" cards the citizen can exchange?