Perform math operation from controller inside ion-content tag

Hi,

I don’t understand <ion-content> tag while doing math operation in controller. Here I’m try to do simple sum operation.
this is work as expedted http://jsfiddle.net/p8eejge1/
in ionic, it’s work only when I remove <ion-content> tag

.controller('BrowseCtrl', function($scope) {

    $scope.n1 = 1;
    $scope.n2 = 1;
    $scope.sum = function() { return $scope.n1 + $scope.n2; }

})

<ion-view view-title="Browse">
  <ion-content>
    <input type = "number" ng-model = "n1"/> +
    <input type = "number" ng-model = "n2"/> =
    <span>{{ sum() }}</span>
  </ion-content>
</ion-view>

what I am missing?