how to subract gridview textbox values based in gridview check checked rows only

hi

i am using asp.net gridview.my gridview contain three textbox and one check box.when the user click the gridview checkbox the particular two textbox value subract and display third textbox.
example:
t1 t2 t3
4 2 2 checkbok click

t2=t1-t3;

please very urgent.help me