adding an extra row in a gridview

Dear all;

I have a gridview which has two columns. The first column is called the date and the second column is called the amount. I would like a situation where I would be able to add an extra row at the end of the gridview and use it to determine the total amount. see example below.

Month                             amount
2001-01                            20
2001-02                            30
2001-03                            40
Total                                 90 (this is the extra row I am talking about). It should be added and include the summation of the amount)

So how do you add this extra row and perform the summation in gridview. Thank you.