How To Auto Subtract Two Text Values

I am trying to change the script below from adding the values to subtracting without much luck.
Code:
<script type="text/javascript" language="javascript">
function autocalc(oText) {
if (isNaN(oText.value)) //filter input {
alert('Numbers only!');
oText.value = '';
} var field, val, oForm = oText.form, netincome = a = 0;
for (a; a < arguments.length; ++a) //loop through text elements {
field = arguments[a];
val = parseFloat(field.value); //get value
if (!isNaN(val)) //number? {
netincome += val; //accumulate
}}
oForm.netincome.value = netincome; //out
}
</script>