For what values of $r$ and $s$ is $f(x,y) = |x|^r|y|^s$ differentiable on $\mathbb{R}^2$?

Question

Let $f(x,y) = |x|^r|y|^s$ where $r$ and $s$ are positive numbers. For what values of $r$ and $s$ is $f(x,y) = |x|^r|y|^s$ differentiable on $\mathbb{R}^2$?