Illustrator Scripting :: Random Delete A Selection Of Items

I am trying to create a script to specify a percentage of the selected items you want to remove.Here is what I have so far.
 
I still need to work out on the alert prompt and the if statement, but for now, I have the feeling that the selection.length keep changing each time Illustrator delete an object, and I don't know how to make it stick.
  
var selection = app.activeDocument.selection;
//alert prompt dialog for a percentage of deletion
for (var i = 0;  i < selection.length;  i++) {
    if (Math.random < 0.5) { //need to work out the percentage
        alert("yes")
        selection[i].remove();
    }
}