Find the exact area of the region under the graph of $y=e^{-x}$ from $x = 0$ to $x = 2$ by using a computer algebra system to evaluate $$\underset{n \rightarrow \infty}{\lim}\, \frac{2}{n}\overset{n}{\underset{i=1}{\sum}} \,e^{-2i/n}$$

Value of limit =