EXCEL recursivity

Good I have the following query, what I want to do is compare the absolute difference between two columns, where one column is full of data and the other lacks some, so the absolute difference of the column where it lacks the same data of the data column. Therefore I would like to know how to program with IF to recursively analyze the rows of these columns and in which there is a difference abs different from the same value that we have in the first column, take it into account while the other does not, the final purpose is to evaluate an average of the total difference. thanks (I have excel in English, but I do not think it matters)