question about expression

[oliviawang]

I'd like to add a baseline.  When the year is 2020, the full year budget is A, when the year is 2021 it's B.

My data is like this :

year  month  fy_budget

2020    1            20

2020    2            20

  ...

2020   12           20

2021    1            70

2021    2            70

...

2021    12           70

How can I write the expression? 

I use: aggr(aggr(avg(fy_budget),year),month),  but it can't work.