function center(){
  if (typeof (window.innerWidth) == 'number') {height = window.innerHeight}
  else if (document.documentElement && document.documentElement.clientHeight) {height = document.documentElement.clientHeight}
  //window.alert ('Height = ' + height)
  margin = (height - 570) / 2
  //window.alert (margin)
  if (margin > 20) {
  	document.getElementById("nagykeret").style.marginTop = margin + "px"
  }
  else {
    document.getElementById("nagykeret").style.marginTop = "20px"
  }
}