Formula Calculation problem Help needed please

Hi All,

I am struggling with a calculation. I need to be able to calculate the total Charge for the YTD which is dependant on the month entered in a cell.

So what I need is when Feb-12 is entered it automatically calculates the totals for Jan and Feb.

I have attached a very basic view of what I need in the hope it helps.

I have tried a sumproduct but can not get it to work.

As always your help is apreciated.