# Have you reached your limit?

Calculus Level 5

$\large\displaystyle\lim_{n \to \infty} \left(e^{-n} \sum_{k=0}^{n} \dfrac{n^{k}}{k!}\right) = \dfrac{a}{b}$

In the above equation, $$a$$ and $$b$$ are positive coprime integers. Find $$a + b$$.

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