Need to calculate systematic skewness

[david55]

Hello

I need to write a formula for my data, but I'm not sure the way I write is the best.

I have two columns of data, let's say in column A and B, which are monthly returns of stock A (Ri) and market (Rm) respectively.

I need to calculate a value similar to covariance but not exactly that.

that is:
formula.jpg


I really appreciate your kind attention.