Calculated Field - Each month variance from opening

Hello, I'm trying to work out the YTD cash movement, for each month, from the opening cash position. Imagine I have an opening balance at 1st Jan of 10. Then by the end of Jan the figure is 15, I want the pivot table to show movement of 5. That I can do. But then, in Feb, the figure is 25 - I want the pivot give me the difference from the OPENING to Feb i.e. 15 and so on for each month. I've tried to show both the data base and what I would like the pivot to show below (period 0 is opening, 1 end of Jan and so on):

DATABASE

Period Balance
0 10
1 15
2 25


PIVOT
Period
0 1 2
Movement 10 5 15


Is there any calculated field or item that would show me each months variance from the same base (the opening)?

Thanks!!

The tables didn't coem out so well...

DATABASE

Period..........Balance
0................10
1................15
2................25


PIVOT
.................Period
.................0......1......2
Movement...10.....5......15


Better??

Thanks for that Izandol, the problem is I'm already using the data field to calculate the YTD figures. So I have it set to running total to give me period 1, then period 1 & 2, then 1 & 2 & 3 and so on. Then it's those YTD figures which I need to calculate the movement to from the opening. Unless I could set the data field to have two criteria?

Otherwise that would have worked.

OK. how do I do that? Sorry - new to all this!