Calculating difference between Inventory for current month and forecast for next months

Hello,

I need help with creating a table like below. It need to get remaining stock in inventory every month till it goes negative. I have given the example below. If current month is August, I take difference between current month inventory and next month sale forecast to get the remaining stock. That is 49218 - 13378 = 35840. For Nov, I take difference from the remaining stock to subsequent month sale Forecast. That is 35840 - 14182 = 21658. I want to do this till I my stock goes negative.

I have tried multiple ways but have not been successful. Any help is much appreciated!

Thanks!