I am confused with this statement
The maximum number of nodes in a binary tree of depth $k$ is $2^k-1$, $k \geq1$.
How come this is true. Lets say I have the following tree
1 / \ 2 3
Here the depth of the tree is 1. So according to the formula, it will be $2^1-1 = 1$. But we have $3$ nodes here. I am really confused with this depth thing.
Any clarifications?