Prove that the function have a saddle point

Question

Let $m,n\in\mathbb{N}$ and $f : \mathbb{R}^2 \rightarrow \mathbb{R}$ be defined by $f(x,y)=x^m - y^n$. Show that $f$ has a saddle point at $(0,0)$ if and only if both $m$ and $n$ are even.