# Mean of two time series

I'm trying to estimate the covariance of two time series using the formula

where X and Y are two time series.
I don't understand how to calculate the value of the first term: does that mean I have to calculate the mean of the means, or do I have to multiply the X serie by the Y serie, and then calculate the mean?
If so, how do you multiply two series?

Forgive me if I'm asking a silly question, I'm new to this subject and it is pretty confusing.
Thanks!

$$\overline{XY}$$ stands for the empirical mean of $$XY$$ where $$XY$$ is a random variable that equals the product of random variables $$X$$ and $$Y$$. That is, $$\overline{XY}=\frac{1}{n}\sum_{i=1}^n x_i y_i$$ for a sample of size $$n$$ where $$x_i$$ is the $$i$$th observation of $$X$$ and $$y_i$$ is the $$i$$th observation of $$Y$$. Hence, you multiply the two vectors element by element, sum them up and divide by the number of elements in one vector.