asp.net programming errors

Hi ,

  I am newer in dot net .i have develop small asp.net
application in this application have three textboxes
and one command button and using c# code
in this  question i am click the button
the third textbox given the sum of two textboxes value
i wrote the some codeing but given error message
is not convert the "string to int' please give me anyone
help

thanks
Raghu