Formula or function for tracking change over time

I am working on a spreadsheet that projects class need and usage over a 4 year period. The top row of my chart tracks how many students are able to enroll each month. I project these enrollments a year at a time then duplicate the numbers for the next 3 years (i.e January is 6, February is 0, March is 4, April is 2, May is 4 and so on).

Students graduate in 5 months. This is tracked in the second line. So January enrollment of 6 and makes a May graduation 6. The third line tracks students in the program in any given month. So if there were 0 before January, and January had 6 enrollments line three would be 6 for January, 6 for February, 10 for March, 12 for April and May would be 10 (6 added/6 graduated).

Is there a way to automate the calculation for the third line? I feel like this should be simple but I can’t quite figure it out.

Editing for relevant info:

I’m online, on a laptop using windows. I’m using Microsoft Office LTSC Professional Plus 2024.