$R=" {" (x, y) : \left | x \right | + \left | y \right | \leq 1"} "$ 일 때, $\displaystyle \iint _ {R} {e^ {x-y}} dA$ 의 값은?

• ①$\displaystyle e- \frac{1} {e}$
• ②$\displaystyle 2e- \frac{1} {2e}$
• ③$\displaystyle \frac{1} {e}$
• ④$\displaystyle e+ \frac{1} {e}$
• ⑤$\displaystyle 2e+ \frac{1} {2e}$