Evaluate$$ \iint_R \left ( e^{-x-y} \right )dxdy, $$ where $R$ is the region in the first quadrant in which $x+y\leq 1$.
I think the first step is $$\int_{0}^{1}\int_{0}^{1-y}\left ( e^{-x-y} \right )dxdy $$but I am not sure about whether this step is correct or not.
Please tell whether my answer is correct or not and please solve it. Thank you.