Dinamic calulations

Hello Every one!

I am having a big toruble with something it seem easy doing in excel, I would appreciate if someone could help me with this, so I have a data set with lost of clients and lots of materials, my data show the aumout of this materials wich the clients buy and sell, I need to track the the inventory the have each week, my data looks like this: 

I create a column with a index to show the order of the weeks acording to the combination of client and material...

what I need to create is this two columns (with calculated columns or measures) wich shows me the inical inventroy of the week and the final inventory (the next incial inventory would be the same as the final inventory as the last week), so for example the first client begin with 0 inventory in the first week, then bought 900 and sell 725, the final inventory for week one would be 0+900-725=175, then 175 will be the inicial inventory for week two, so the calculation for week two is going to be (175+2050-1400=825).

The result I want is this two columns to show like this: 

 I think i need a variable to save the calc. between buy and sell and bring it to the next period

I want to share my file, if someone knows how to share a file in this forum would be great

I really need some help!

Thanks