footer calucation in asp.net gridview

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ASP.NET

hi all, i have created invoice program in asp.net, in my sales invoice i have grid, in my grid user wil enter item,price, quantity, when user enter quantity i need to multiply the price * quantity and the result should come in total, how to do this?

note :-
user enter item,price,and quantity in gridview footer textbox,also result comes in footer textbox itself.