\lambda e^{-\lambda}\displaystyle\bigg[\sum_{x=1}^\infty \frac{\lambda^{x-1}}{(x-1)!}\bigg+\displaystyle\bigg\sum_{x=1}^\infty (x-1)\frac{\lambda^{x-1}}{(x-1)!}\bigg]=...=\lambda+\lambda^2