The definition of full binary tree is "a tree in which every node other than the leaves has two children" (

Wikipedia). If you draw them on paper, you will notice that you start out with just one vertex, which we can call a leaf, then after that each leaf creates two new leaves while becoming itself an internal vertex. So, each time you add two branches to a leaf, you destroy one leaf and create two more. The overall effect is that the number of leaves is always one greater than the number of internal vertices. So, we could write

n + (n+1) = 231