How to sum the next n non-empty cells in LibreOffice Calc

Question

My LibreOffice Calc spreadsheet has cells with values in an atypical pattern in one column, which means that any number of empty or non-empty cells may follow a non-empty cell.

How can I add the following n (for example, 3) non-empty cells?

I discovered that the COUNTA function, an IF condition, and a dynamic range will likely require the SUM function. I can't seem to put it all together, though. With =SUM(B2:INDEX(B:B,COUNTA(B:B)) for instance, I can add up all non-empty rows in B, but how do I narrow this down to the following 3 non-empty entries?

Answer 1

Here's a volatile solution (assuming you don't have more than 10k rows): 

The array formula in G2 is

=SUM(INDEX(B2:$B$10000,AGGREGATE(15,6,(1/ISNUMBER(B2:$B$10000))*(ROW(B2:$B$10000)-(ROW(B2)-1)),ROW(INDIRECT("1:"&D2)))))

(you could possibly reduce volatility by hard-coding the array constant - instead of using INDIRECT() - but I'm not very familiar with all of LibreOffice's volatile functions)