Adding row to datagridview

Question

Hi All,
In my application, i have a datagridview with 5 colums, at the form load the datagridview contains one row, I am showing a combobox in each of the 4 columns to select the values (i doesnot type any thing in that 4 columns), when i press tab the focus goes from column 4 to column 5.
Now when again i press tab the focus leaves the datagridview, but here i want that when i leave the column 5, there must be a row added.
How can i do that, can anyone help me on that?
Thanks in advance.
Regards,
Vidya