Heap-like notation of a BST (BFT bit-vectors) for succinct representation
You can count it like this:

Let $n$ be the original number of nodes

Each original node has 2 children after adding the external nodes. 

So 1 bit for the original node and 2 for its children. Total $3n$ bits.

But we are over counting here because original node is child of one other original node. So we subtract $n$ and get $2n$. 

But we over corrected the count because root node has no parent so we add back 1 bit. 

$2n + 1$.