The probability mass function of random variable $X$ is given by $$P(X=i)=e^{-2}\*\frac{2^{i}}{i!}, i=0,1,2,....$$ Find the expectation, $E[X]$.

I know that, 
$$E[X]=\sum\_{x, p(x)>0}xp(x)$$

I am just struggling understanding the problem.