Edit in JSFiddle

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

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