Allow calculating negative numbers in form fields

This might be a javaScript question, but I am sending entries from a form via PHP to a MySQL db.  Sometimes I beed to be able to enter a negative numeric value (-56.00, or -.63) but I can't seem to find a way to calculate any of these fields without the negative numbers being surrounded in brackets '()' and the result displaying 'NAN' in the total fields.  Any ideas?

Thanks for your help!

Doug

Code:

function calcincome(form, elemName, ttl) {

      form.currency_total.value = Fix(eval(form.currency_am.value-0) + (form.currency_pm.value-0));

}

<form name="smartform">

<input type="text" name="currency_am" value=""  id="currency_am" onChange="calcincome(this.form,'currency_am','Currency for Day Shift'); formatNumber(this, 2);"  />

<input type="text" name="currency_pm" value=""  id="currency_pm" onChange="calcincome(this.form,'currency_pm','Currency for Day Shift'); formatNumber(this, 2);"  />

<input type="text" name="currency_total" value="" onChange="this.form.currency_am.value + this.form.currency_pm.value; calcincome(this.form,'field'); formatNumber(this, 2);" />

</form>