$\displaystyle f_{X,Y}(x,y) = \{x + y , 0 \leq x \leq 1, 0 \leq y \leq 1; 0 otherwise$

Find $\displaystyle Pr(X+Y \leq 1)$

I understand i need to do $\displaystyle \int \int (x+y) dy dx$

But i can't work out what the limits of the integrals should be, any help would be great, thanks.