cumulative of stock qty

Dear All,

I want to keep track on stock qty which one sold or in hand, example : i purchased 50 numbers of stock item, it should be numbered like 1 - 50, then again i purchased 50 numbers, then number should be 51-100. when i sold the stock it should pick from one of these number only and stock has reduced accordingly.

unable to get any idea, how to do this. can you please give some idea / sample code.

Thanks & warm regards

 

But Sir,
numbering should be automatic, and next adding qty also should be auto numbering and it should be sequence.

 

while entering the sales voucher, this number should come first, then stock item name

 

Any Updates on this.... Please help me