Stop to add points in a polygon with openlayers 5.3.0

In my web page, users can draw a polygon and then modify it. However, I would like to only allow users to drag and drop the drawn corners, not add new points into polygon when a polygon is modified.

In the example below, I created a polygon with four corners and would like to keep four corners during modifying (only drag the corners).

I think we should use condition in the modify function, but not sure how to find out the difference between clicking the corner or edge of a polygon.

<!DOCTYPE html>
<html>
  <head>
    <title>Draw Features</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v5.3.0/css/ol.css" type="text/css">
      <script src="https://cdn.rawgit.com/openlayers/openlayers.github.io/master/en/v5.3.0/build/ol.js"></script>
  </head>
  <body>
    <div id="map" class="map"></div>
    <script>
      var raster = new ol.layer.Tile({
        source: new ol.source.OSM()
      });


      var source = new ol.source.Vector({wrapX: false});
      var offset = 1000000;
      var ply = new ol.geom.Polygon([[
        [-11000000 - offset, 4600000 - offset], 
        [-11000000 + offset, 4600000 - offset], 
        [-11000000 + offset, 4600000 + offset], 
        [-11000000 - offset, 4600000 + offset]]]);
      var feature = new ol.Feature(ply);
      source.addFeatures([feature]);
      var vector = new ol.layer.Vector({
        source: source
      });

      var map = new ol.Map({
        layers: [raster, vector],
        target: 'map',
        view: new ol.View({
          center: [-11000000, 4600000],
          zoom: 4
        })
      });

      var modify = new ol.interaction.Modify({source: source})
      map.addInteraction(modify);
    </script>
  </body>
</html>

Solution

 var modify = new ol.interaction.Modify({
     source: source,
     insertVertexCondition: ol.events.condition.never
 });