Web Forms :: Sum Value Of Cell Based On CheckBox In GV

I have a checkbox in GV as (template) [ its index 0] and lblsum (lblsum not in GV ) and I need :

1- if i check checkbox get value of cell [1] and add it to value in lblsum (for each row in GV)
2- if i uncheck checkbox subtraction value of cell[1] from lblsum

This code not work

 protected void chkCtr2_CheckedChanged(object sender, EventArgs e) {
CheckBox chkRow = (row.Cells[0].FindControl("chkCtr2") as CheckBox);
if (chkRow.Checked)
{ int myval = int.Parse(e.Row.Cells[1].Text);
slblsum.text=int.parse(lblsum.text)+ myval
}
else
{int myval = int.Parse(e.Row.Cells[1].Text);
slblsum.text=int.parse(lblsum.text)- myval
}