How to calculate amount in gridview column

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Gridview

i have gridview in which there 4 cloumns like dropdownlist which shows products, Rate Column, Quantity Column and Total Amount Column i want that when i enter rate and quantity total amount show automatically count.
kindly guide me

while i also tried code in gridview rowdatabound but it shows string was not in correct format

What I have tried:

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('-', ',')));

            }