Having trouble setting up the (I assume) double integral for the following:

$\displaystyle P[(X\geq m) \cap (X - m)>(Y-n)]$

where

$\displaystyle X,Y$ follow a joint uniform distribution as follows:

$\displaystyle f(x,y)=\frac{1}{a^{2}}$ for $\displaystyle 0\leq X\leq a , 0\leq Y\leq a$

Any help would be appreciated. Thanks!