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// based on knowledge by redefinery http://www.redefinery.com/ae/fundamentals/masks/
// and the AECS6 scripting guide also written by redefinery
var layer = app.project.activeItem.selectedLayers[0];

var masksGroup = layer("Masks");
if (masksGroup != null){
    
    var my_maskShape = layer.mask(1).property("ADBE Mask Shape").value;
    
    alert (my_maskShape.vertices.length);

}