calculate the checked rows data in gridview using asp with c#

Question

hi , iam using asp.net with c#
in my gridview at the form load iam displaying data, 1st column is product name, 2d column is price,
3rd column is quantity, 4th column is total. in total colum iam using gridview _rowdatabound event and calculating the grand total. but i want when user check or unckeck the checkbox then i want that rows total as grand total
can you give example which helps me