In fact, you can determine what that sum is equal to. It is well known that . Here you have
If this power series isn't known to you (which it really should be) we can find it slighly differently. Suppose we just want to find the explicity function described by notice that we may (take for granted here that you can...there is criterion it needs to meet but it does...so) differentate to obtain . Therefore, . And seeing that we can conclude that therefore .