flex dataGrid中的计算问题

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
  layout="absolute">
  <mx:Script>
  <![CDATA[
  import mx.controls.Alert;
  import mx.controls.CheckBox;
  import mx.events.FlexEvent;
  import mx.collections.ArrayCollection;
  import mx.events.AdvancedDataGridEvent;
  [Bindable]
  private var dataSource:ArrayCollection=new ArrayCollection([
  {id: 1,dengyu:'='}, 
  {id: 2,dengyu:'='}, 
  {id: 3,dengyu:'='}, 
  {id: 4,dengyu:'='}, 
  {id: 5,dengyu:'='},
  {id:'合计：',num1:'',num2:'',dengyu:'---',sum:''}
  ]);

   
  ]]>
  </mx:Script>

  <mx:AdvancedDataGrid id="adGrid" sortExpertMode="true"
  width="300"
  height="253"
  fontSize="14"
  dataProvider="{dataSource}"
  x="262.5"
  y="2