function getWidth() {
  return Math.max(
    document.body.scrollWidth,
    document.documentElement.scrollWidth,
    document.body.offsetWidth,
    document.documentElement.offsetWidth,
    document.documentElement.clientWidth
  );
}

if (getWidth() < 750) {
    gremi.advert.modify({
            advertId:   document.currentScript.parentNode.id,
            scale:      false, // change to 'true' to enable advert scaling
            height:     300, // change to original ad height if you enabled scaling
            width:      750, // change to original ad width if you enabled scaling
            addLabel:   true,  // change to 'false' if you want to remove 'REKLAMA' label
            addMargins: true,  // change to 'false' if ad should take no space on site by default
            remove:     false  // change to true to remove any trace of placement from site
    });
}