OpenLayers live circle radius

HTML

<link rel="stylesheet" href="http://openlayers.org/api/theme/default/style.css">
<script src="http://openlayers.org/api/OpenLayers.js"></script>
<link rel="stylesheet" href="http://dev.openlayers.org/releases/OpenLayers-2.13.1/examples/style.css">
<script src="http://web-mapping.com/test/RegularPolygon_thomas.js"></script>
 <h1 id="title">Live Measurement of Circle Radius</h1>

<div id="tags">drawing</div>
<p id="shortdesc">Draw a circle to see its live radius.</p>
Radius: <span id="radius">xxx</span> <span id="einheit">Meter</span>
<div id="map" class="smallmap"></div>

CSS

#radius
{
    color:red;
    font-weight:bold;
    
}

JavaScript

//initialize two vector layers
var circles = new OpenLayers.Layer.Vector("Circles");
var radii = new OpenLayers.Layer.Vector("Radii");

//initialize a draw control
var my_polygonhandler=OpenLayers.Handler.RegularPolygon;

var polygonControl = new OpenLayers.Control.DrawFeature(circles,
my_polygonhandler, {
    handlerOptions: {
        sides: 40
    }
});

console.log(polygonControl);
//initialize a map
var map = new OpenLayers.Map({
    div: 'map',
    projection: new OpenLayers.Projection('EPSG:900913'),
    displayProjection: new OpenLayers.Projection('EPSG:4326'),
    layers: [
    new OpenLayers.Layer.OSM(),
    circles,
    radii]
});
if (!map.getCenter()) {
    map.zoomToMaxExtent();
}
map.addControl(polygonControl);
polygonControl.activate();

circles.events.on({
    'featureadded': function (e) {
        
        var f = e.feature;
        //calculate the min/max coordinates of a circle
        var minX = f.geometry.bounds.left;
        var minY = f.geometry.bounds.bottom;
        var maxX = f.geometry.bounds.right;
        var maxY = f.geometry.bounds.top;
        //calculate the center coordinates
        var startX = (minX + maxX) / 2;
        var startY = (minY + maxY) / 2;

        //make two points at center and at the edge
        var startPoint = new OpenLayers.Geometry.Point(startX, startY);
        var endPoint = new OpenLayers.Geometry.Point(maxX, startY);
        var radius = new OpenLayers.Geometry.LineString([startPoint, endPoint]);
        //calculate length. WARNING! The EPSG:900913 lengths are meaningless except around the equator. Either use a local coordinate system like UTM, or geodesic calculations.
        var len = Math.round(radius.getLength()).toString();
        //style the radius
        var style = {
            strokeColor: "#0500bd",
            strokeWidth: 3
            //,label: len
        };
        //add radius feature to radii layer
        document.getElementById("radius").innerHTML=len;
        var fea = new...