Gridview paging

Question

Hi all, I am displaying the sales table data in grid view with field like itemid, itemname, quantity, cost. I have set paging, each page can hold 10 rows. In each page i want to calculate total quantity and TotalCost in two text box. When i am running the application by default in page1, in txtquanty and txtTotal cost should display the total quantity and total cost. When user click on the page2 it should display totalquantity and totalcost...it has to continue....dynamically......Pls give me the solution how do achieve it????

Thanks, Sumit