HFM Rules file updates

Hi everyone,

I'm trying to modify the HFM rules to calculate the Custom3=Alloc YTD data of current period with the sum of YTD amount of previous month and Periodic amount of current month. Appreciate if anyone can provide inputs on how to write this logic in HFM rules file. Thanks in advance.

Ex: Current month = Feb

YTD of previous month: FCST04;2017;Jan;YTD;NonOpAlloc;<Entity Currency>;93800;[ICP None];[None];[None];Alloc;[None];10000

Periodic of current month: FCST04;2017;Feb;Periodic;NonOpAlloc;<Entity Currency>;93800;[ICP None];[None];[None];Alloc;[None];20000

Result:

YTD of current month: FCST04;2017;Feb;YTD;NonOpAlloc;<Entity Currency>;93800;[ICP None];[None];[None];Alloc;[None];