Need a Formula to sum across columns for date + 14 months

Please see attached file with sample data. In cell AL3 and AL4, I manually entered a formula to add the first 14 months of purchases and discounts across. Now in the next row, 5, I want the formula to start the addition in cell E5 and add 14 (exactly 14, not more, not less) columns across and display the result in cell AL5. The pattern should continue as long as there is data in the rows. Instead of doing it manually row by row, I’m hoping some formula will make it easier. Thanks in advance for your help!