Using store data in database with asp.net

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ASP.NET

I am trying to save some records to store a table through front end.
here i am selecting product_name from dropdownlist there rate display as selected dropdownlist from database now inserting quantity fron text box and another text box calculate amount and then using save button I am getting error

system.formatexception: failed="" to="" convert="" parameter="" value="" from="" a="" string="" int32.="" input="" was="" not="" in="" correct="" format.="" at="" numberstyles="" numberformatinfo="" boolean="" type="" iformatprovider="" metatype="" ---="" end="" of="" inner="" exception="" stack="" trace="" sqlparametercollection="" runbehavior="" dbasyncresult="" eventargs="" 102=""

and my source code is..

 protected void btnSubmit_Click(object sender, EventArgs e)
        {
            SqlConnection con=new SqlConnection(ConfigurationManager.AppSettings["connectionstring"].ToString());
            SqlCommand cmd;
            SqlDataAdapter da=new SqlDataAdapter();
            DataSet ds=new DataSet();
            con.Open();
            try
            {
                cmd=new SqlCommand("insert into product(product_id,product_name,product_rate,product_quantity,product_total_amount)values(insert into product('@product_id',@product_name,@product_rate,@product_quantity,@product_total_amount)",con);
                cmd.Parameters.Add("@product_id", SqlDbType.Int).Value = DropDownList1.SelectedItem.Text;
                cmd.Parameters.Add("@product_name", SqlDbType.NVarChar).Value = DropDownList1.Items.ToString();
                cmd.Parameters.Add("@product_quanity", SqlDbType.NVarChar).Value = txtqty.Text;
                cmd.Parameters.Add("@product_rate", SqlDbType.NVarChar).Value = LblRate1.TemplateControl;
                cmd.Parameters.Add("@product_total_amount", SqlDbType.NVarChar).Value = txtTotalAmount.Text;
                cmd.ExecuteNonQuery();
                Response.Write("yr records updated..");
                da = new SqlDataAdapter("select * from product",con);
                GridView1.DataSource = ds;
                GridView1.DataBind();
             }
            catch(Exception exe)
            {
                Response.Write(exe);
            }
           // cmd.Dispose();
            ds.Dispose();
            con.Close();            
}

what is the wrong input in this code. Please tell me?