Displaying running totals in a GRIDVIEW asp.net 2.0

Question

Dear all,

I got a gridview with template columns
col 1: ddlist with Article type
col 2:Quantity a textbox which is entered by the user
col 3:Rate per Item a textbox which is entered by the user
col 4:Total which is a Label =col2 * col3; which must be automatically displayed when control goes to  next row.....

and there is a label in the footer of the gridview which should display the total of the col4:

which event of the gridview we must write the code...

AMR