Multiply two columns in gridview

Backend :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.Text;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {

            string strcon = ConfigurationManager.ConnectionStrings["ConnectionStringdb"].ConnectionString;
            using (SqlConnection con = new SqlConnection(strcon))
            {

                SqlCommand cmd = new SqlCommand("USP_datamanipulation", con);
                cmd.CommandType = CommandType.StoredProcedure;

                con.Open();
                GridView1.DataSource = cmd.ExecuteReader();
                GridView1.DataBind();
                con.Close();
            }


        }
    }
    protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
    {


        for (int i = 0; i < GridView1.Rows.Count; i++)
        {
       
            GridView1.Rows[i].Cells[3].Text = Convert.ToString(Convert.ToDecimal(GridView1.Rows[i].Cells[1].Text) * Convert.ToDecimal(GridView1.Rows[i].Cells[2].Text));

        }

    }
}

o/p