Thank you so much Ron and Biff, both solutions work pretty well.
Nevertheless I'm encountering one more difficulty, my data set is
really large (+400,000 rows) so pasting down any of these formulas is
not practical because as I paste them down, they stay fixed to the
first cell of the range. However, I know that the data is ordered, and
that in any case, if I were able to look just 100 rows forward and 100
rows ahead, that would be enough. I've been trying to fix both
formulas with ADDRESS() in such a way that I could vary the initial
and ending rows in such way, but I haven't been able to do so
effectively. Any thoughts? Thank you again.

JM