How to insert a new item into spreadsheet and have the balance correct?

Here is a partial view of a spread sheet I am currently using and am quite happy. However, I realized I forgot to add a couple of items between March 13 and March 18. If I add new rows where they should chronologically go, it seems to mess up the formula and the balance column no longer works. Is there an easy way to fix this so the formula will include the correct new balance after I add the two new items. I realize I could add them at the end but I really would like them to be chronologically correct when I submit this spreadsheet of purchases. Thank you for any help. I'm a Numbers newbie.