add operation on multiple rows specific cells of DataGridVie

Question

Hi,
I have one Grid with columns Id,Name,Balance.Here is my DatagGridView.Where Cr. and Dr. are Credit and Debit.

Id        Name     Balance
1           Sai        10000Cr.
2           Ram      3000Dr.
3          Anu        5000Cr.
4         Swathi     4000Dr.
And I have a TextBox outside of GridView,I want to do sum operation on the above 4 values and the result(8000Cr) should be stored in TextBox.Here I didn't use CheckBox and which DataGridView click event we will write the code?
Thanks in advance.