how to calculate total in in gridview column

Question

hi iam using asp.net with c#
in my gridview 1st column is checkbox
select      productname      price     quantity    total
()           rose                    124          6          124*6
()          biscuits                  345       89         345*89
()           choclate                 670       45         670*45
                                                                    =======
                                                                        67000
                                                                      ========   

when user enters quantity automatically in total colum  total is displayed
please can you give some example for this ...