function change_display(id){
  if(document.getElementById(id).style.display=='none'){
    document.getElementById(id).style.display='block';
  }else{
    document.getElementById(id).style.display='none';  
  }
}

function vypis_cas(){
  sekundy++; if(sekundy==60){ sekundy=0; minuty++; }

  vypis=''; 
  if(minuty<10) vypis+='0';
  vypis+=minuty+':';
  if(sekundy<10) vypis+='0'; 
  vypis+=sekundy;

  document.getElementById('casomiera').innerHTML=vypis;

  if(minuty==limit) document.location=document.location+'';
}

function getHeight(){
  var viewportheight;
  
  if (typeof window.innerHeight != 'undefined'){
    // the more standards compliant browsers (mozilla/netscape/opera/IE7) use window.innerWidth and window.innerHeight   
    viewportheight = window.innerHeight
  }else if (typeof document.documentElement != 'undefined' && typeof document.documentElement.clientHeight != 'undefined' && document.documentElement.clientHeight != 0){
    // IE6 in standards compliant mode (i.e. with a valid doctype as the first line in the document)  
    viewportheight = document.documentElement.clientHeight
  }else{
    // older versions of IE
    viewportheight = document.getElementsByTagName('body')[0].clientHeight
  }

  return viewportheight;
}