How to calculate the sum of the fields in Gridview

protected void btnSave_Click(object sender, EventArgs e)
{
    ResultDbEntities rd = new ResultDbEntities();

    tblPE1 pe1 = new tblPE1();
    rd.tblPE1.Add(new tblPE1() { StID = int.Parse(txtID.Text), SName = txtName.Text, FName = txtFName.Text, Group = txtGroup.Text,English=int.Parse(txtEng.Text), Physics=int.Parse(txtPhy1.Text), Chemistry=int.Parse(txtChem1.Text),Math=int.Parse(txtMath.Text),term=txtTerm.Text,});
    
    rd.SaveChanges();
    
    var res = re.tblPE1.ToList();

    GridView1.DataSource = res.ToList();
    GridView1.DataBind();
}