Summation with factorial

I want to understand how this step is performed.

Can you tell me that how this value of Po is obtained from the first equation.!

$$P_o=\frac{1}{\sum_{k=0}^\infty\left(\frac{\alpha}{u}\right)^k\frac{1}{k!}}$$

$$P_o=e^{-\frac{\alpha}{u}}$$