How to check the checkbox checked or not in GridView

int tot=0;

foreach (GridViewRow r in GridView1.Rows)

{

CheckBox chkBox = new CheckBox();

chkBox = ((CheckBox)GridView1.FindControl("checkbox1"));

if ( chkBox.Checked)

tot = tot + int.Parse(r.Cells[4].Text);

}

Label1.Text="toal" + tot;