Untitled

<html>
<head>
<title>Test</title>
<script type="text/javascript">
function ausrechnen() {
var ziffer1 = parseInt(document.querySelectorAll('input[type=number]')[0].value),
    ziffer2 = parseInt(document.querySelectorAll('input[type=number]')[1].value);

document.querySelector('input[name=endErgebnis]').value = ziffer1+ziffer2;
}

document.addEventListener("DOMContentLoaded", function() {

var button = document.querySelector('input[type=button]');
button.addEventListener('click', ausrechnen);

}, false);


</script>
</head>
<body>
<form name="Addition" action="">
<input type="number"   name="Ziffer1">
<input type="number"   name="Ziffer2">
<input type="button"   value="Addieren">
<input type="number"   name="endErgebnis" value="" readonly>
</form>
</body>
</html>