Move background image based on mouse position from right side of screen

Hi

Im moving a background image based on x mouse position with :

$("html").mousemove(function(e){
  var strength1 = 100;
  var pageX = e.pageX - ($(window).width() / 2);
  $('#foreground1').css("background-position", (strength2 / $(window).width() * pageX * -1));
}

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this works fine with images on left. How can i adjust this for an image sat on the right of the screen?

Thanks