function berekenTotaal(prijs, elAantal, elLabel)
{
     var ob = document.getElementById(elLabel);
     var elA = document.getElementById(elAantal);
    if ((ob != null) && (elA != null))
    {
        alert(prijs);
        ob.value = '€ ' + (prijs * elA.value).toString();
        ob.innerText = '½ ' + (prijs * elA.value).toString();
    }
}