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<div ng-app="demo">
  <div ng-controller="MainCtrl">
    <input type="number" ng-model="radius"/>
    <div>Area {{area}}</div>
  </div>
</div>

<script>
  angular.module('demo', [])
    .controller('MainCtrl', function($scope) {
      $scope.radius = 1;
      $scope.$watch('radius', function (radius) {
        var result = Math.PI * $scope.radius * $scope.radius;
        console.log("radius changed, area = " + result);
        $scope.area = result;
      });
    });
</script>