Add a Button
<asp:Button ID="btnAddPaidAmount" runat="server" OnClick="SumPaidAmount" Text="Sum Paid Amount" />
And a Label
<asp:Label ID="lblSum" Text="" runat="server" />
And the following code
protected void SumPaidAmount(object sender, EventArgs e)
{
int sum = 0;
foreach (GridViewRow row1 in GRDMonthly.Rows)
{
sum += int.Parse(((TextBox)(row1.Cells[3].FindControl("txtMarks"))).Text);
}
lblSum.Text = sum.ToString();
}