\begin{eqnarray*} P(X=x) = \frac {\lambda^{x} e^{-\lambda}}{x!}, \;\; \text{for } x = 0, 1, 2, 3, . . . \end{eqnarray*}

e <- exp(1) 
e

> e <- exp(1) 
> e
[1] 2.718282