mapy google uzyskują promień

Mam głupi trud z tym kodem Google Maps javascript. Chcę uzyskać zwrot promienia okręgu, który jest tworzony na mapach google. Interfejs API Google oferuje kod, ale nie mam pojęcia, gdzie go umieścić. Próbowałem wszędzie. Oto kod poniżej. Co zrobiłem źle?

<!DOCTYPE html>
<html>
<head>

<meta name="viewport" content="initial-scale=1.0, user-scalable=no">

<meta charset="utf-8">

<style type="text/css"> 
 html { height: 100% }
 body { height: 100%; margin: 0px; padding: 0px }
 #map_canvas { height: 50% }
</style> 

<script type="text/javascript"
  src="http://maps.googleapis.com/maps/api/js?key=mykey&sensor=false&libraries=drawing">
</script>

<script>
  function initialize() {
    var mapOptions = {
      center: new google.maps.LatLng(33.65, -84.42),
      zoom: 7,
      mapTypeId: google.maps.MapTypeId.ROADMAP
    };

    var map = new google.maps.Map(document.getElementById('map_canvas'),
      mapOptions);

    var drawingManager = new google.maps.drawing.DrawingManager({
      drawingMode: google.maps.drawing.OverlayType.MARKER,
      drawingControl: true,
      drawingControlOptions: {
        position: google.maps.ControlPosition.TOP_CENTER,
        drawingModes: [

          google.maps.drawing.OverlayType.CIRCLE
        ]
      },

      circleOptions: {
        fillColor: '#01939A',
        fillOpacity: .4,
        strokeWeight: 5,
        clickable: false,
        editable: true,
        zIndex: 1
      }
    });




    drawingManager.setMap(map);





     }

  google.maps.event.addDomListener(window, 'load', 

  initialize).addListener(drawingManager, 'circlecomplete', function(circle) 

  {

  var radius = circle.getRadius();

  });

</script>

</head>

<body>

<script type="text/javascript" >

        alert(radius);

</script>


<div id="map_canvas"></div>

</body>

</html>

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