Claude. To be fair, it had trouble OCR-ing it. After way too much back and forth about the pattern, it produced this:

=LET(
    rows, SEQUENCE(INT((667-30)/13+1),1,30,13),
    SUM(INDEX(AA:AA,rows))
)

Which is correct, but obviously the calculations would be best served by reorganizing the table and probably using a pivot table or at least grouping and subtotaling.