function addListeners(e) {
	// VOEG HIER ALLE EVENTLISTENERS TOE
	var selectMateriaal = document.getElementById('materiaal');
	addEvent(selectMateriaal, 'change', changeAmount, false);
}	
	
addEvent(window, 'load', addListeners, false);

function addEvent(elm, evType, fn, useCapture)
//cross-browser event handling (IE5+, NS6+, Mozilla/Gecko)
{
	if(elm.addEventListener) {
		elm.addEventListener(evType, fn, useCapture);
		return true;
	} else if (elm.attachEvent) {
		var r = elm.attachEvent('on' + evType, fn);
		return r;
	} else {
		elm['on' + evType] = fn;
	}
}