I am learning Angular using W3Schools.

I just modified an example about "Services"... The following is the code:

```html
<!DOCTYPE html>
<html>
<script src="http://ajax.googleapis.com/ajax/libs/angularjs/1.4.8/angular.min.js"></script>
<body>
<div ng-app="myApp" ng-controller="myCtrl">

<p><input type="text" ng-model="num"></p>
<h2>{{num}}</h2>
<h1>{{hex}}</h1>

</div>

<p>A custom service whith a method that converts a given number into a hexadecimal number.</p>

<script>
var app = angular.module('myApp', []);

app.service('hexafy', function() {
    this.myFunc = function (x) {
        return x.toString(16);
    }
});
app.controller('myCtrl', function($scope, hexafy) {
  $scope.num = 200;
  $scope.hex = hexafy.myFunc($scope.num);
});
</script>

</body>
</html>
```

When I update the textbox, the "HEX" part is not updating. Why?