A farmer has two types of trees: apple trees and orange trees, planted in a sequence around his orchard. The sequence begins with 1 apple tree, followed by 2 orange trees, then 3 apple trees, 4 orange trees, and so on, increasing by one tree each time. If the sequence continues in this pattern, what will be the total number of trees when there are 7 apple tree groups? Let's think step by step and output the final answer within \boxed{}.
user