window.onload = setOnClicks;

function UpdateCounter(trgtobj,value){
//var cena=200;
//var value=parseInt(value);
var value=parseInt(value);
var cena=value;
var obj = document.getElementById(trgtobj);
var CheckBoxCounter = document.forms['itemsform'].CheckBoxCounter;
if(obj.checked == true)
{
  cena = CheckBoxCounter.value;
  cena = (parseInt(cena,10)+ value);
  CheckBoxCounter.value=cena;
}
else
{
  cena = CheckBoxCounter.value;
  cena = (parseInt(cena,10)- value);
  CheckBoxCounter.value=cena;
}
}

function setOnClicks() 
{

  var inputs = document.getElementById('itemsform').getElementsByTagName('input');
  for (var i=0; i<inputs.length; i++) 
  {
    if (inputs[i].type=='checkbox') 
    {
      inputs[i].onclick = function() { UpdateCounter(this.id,this.value) }
    }
  }

}