Joint CDF to Joint PDF

$F_{X,Y}(x,y)={\int }_{-\infty }^x{\int }_{-\infty }^y{f}_{X,Y}(s,t)dtds,\forall x,y\in \mathbb{R}$ Or, $f_{X,Y}(x,y)=\frac{{\partial }^2}{\partial x\partial y}{F}_{X,Y}(x,y),\forall x,y$