try the code below:
<input type="text" id="input1" />
<input type="text" id="input2" onchange="doMath();"/>
<input type="text" id='my_sum' readonly />
<script type="text/javascript">
function doMath()
{
var input1 = document.getElementById('input1').value;
var input2 = document.getElementById('input2').value;
var sum = parseInt(input1) + parseInt(input2);
document.getElementById('my_sum').value = sum;
}
</script>