如何通过从ng-repeat值中减去值来设置列值

<div ng-app="myApp" ng-controller="myCtrl">
  <table>
    <tr>
      <th>编号</th>
      <th>数值</th>
      <th>减去值</th>
      <th>列值</th>
    </tr>
    <tr ng-repeat="item in items">
      <td>{{$index + 1}}</td>
      <td>{{item.value}}</td>
      <td>{{item.subtract}}</td>
      <td>{{item.value - item.subtract}}</td>
    </tr>
  </table>
</div>

<script>
  var app = angular.module('myApp', []);
  app.controller('myCtrl', function($scope) {
    $scope.items = [
      {value: 10, subtract: 5},
      {value: 20, subtract: 8},
      {value: 15, subtract: 3}
    ];
  });
</script>