How to multiply two columns in gridview using template field

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ASP.NET4

i have 4 columns in gridview like this products, rate, quantity, amount

i want how to calculate amount after rate and quantity multiply i am using template field

kindly, help me i am new

What I have tried:

<asp:GridView ID="GridView1" runat="server" ondatabound="GridView1_DataBound"
onrowdatabound="GridView1_RowDataBound"
onselectedindexchanged="GridView1_SelectedIndexChanged">
<columns> <asp:TemplateField HeaderText="Item Name">
<itemtemplate>
<asp:DropDownList ID="cbGvProd" runat="server" Height="16px" Width="200px">



<asp:TemplateField HeaderText="Quantity">
<itemtemplate>
<asp:TextBox ID="tbQty" runat="server">


<asp:TemplateField HeaderText="Rate">
<itemtemplate>
<asp:TextBox ID="tbRate" runat="server">


<asp:TemplateField HeaderText="Amount">
<itemtemplate>
<asp:TextBox ID="tbAmount" runat="server">



for (int i = 0; i < GridView1.Rows.Count; i++)
{


GridView1.Rows[i].Cells[3].Text = Convert.ToString(Convert.ToInt32(GridView1.Rows[i].Cells[1].Text.Replace('-', ',')) * Convert.ToDouble(GridView1.Rows[i].Cells[2].Text.Replace('-', ',')));

}