calculated columns in grid automatically.

Hi tried with below code, but getting error 'Conversion from string "return show('" to type 'Double' is not valid.'
please help...
vb code:
Private Sub GD_Prod_ItemCreated(ByVal sender As Object, ByVal e As Telerik.Web.UI.GridItemEventArgs) Handles GD_Prod.ItemCreated
If TypeOf e.Item Is GridDataItem Then
Dim item As GridDataItem = DirectCast(e.Item, GridDataItem)
Dim txtbox1 As TextBox = DirectCast(item.FindControl("TxtSTUPort"), TextBox)
Dim txtbox2 As TextBox = DirectCast(item.FindControl("TxtAAPort"), TextBox)
txtbox1.Attributes.Add("onblur", "return show('" + item.ItemIndex & "')")
txtbox2.Attributes.Add("onblur", "return show('" + item.ItemIndex & "')")
End If
javascript:
function show(index) {
var grid = $find("<%=GD_Prod.ClientID %>");
var MasterTable = grid.get_masterTableView();
var row = MasterTable.get_dataItems()[index];
var Value1 = row.findElement("TxtSTUPort").value;
var Value2 = row.findElement("TxtAAPort").value;
var cellValue = parseInt(Value1) + parseInt(Value2);
MasterTable.getCellByColumnUniqueName(row, "STD_NUM_OF_SERVING").innerHTML = cellValue;
}