= e^{ - \lambda } \lambda \left[ {\sum\limits_{x = 1}^\infty {\left( {x - 1} \right)\frac{{\lambda ^{x - 1} }}
{{\left( {x - 1} \right)!}}} + \sum\limits_{x = 1}^\infty {1 \cdot \frac{{\lambda ^{x - 1} }}
{{\left( {x - 1} \right)!}}} } \right]