Calculate monthly production needs based on aggregate production table

Hello all,

I have a table showing aggregate production needs for 2021 based on sales forecast and end of month available inventory. I need to calculate monthly production needs using that table.

For example, at the end of February, I will have a deficit of 770 units of SKU 1. But since I had 166 units on hand in January, I only need to produce 604 units in February.

My question is how to write a formula that properly calculates that need despite having negative numbers in the monthly columns. For SKU 1 in February, I would write =D4+C4 since SKU 1 is negative, but that formula doesn't work for SKU 2 in February. I think I need the difference between each month and its prior month's value.

Thanks in advance!