Calculation script runs every time I enter a value in any field

I am using the following script to multiply a value entered by the user times -1. It also changes a zero to a space.

// compute the discount amount based on the discount rate
event.value = this.getField("spouse_immediate_lifestyle").value * this.getField("negative_multiplier").value;
// suppress a zero result
if (event.value == 0) event.value = "";

I enter a value in cell one and it calculates properly, but when I enter a value in cell two, the negative in cell 1 changes to a positive. Then when I enter a value in cell 3, cell one changes back to negative and cell 2 (same script with modified field name) changes to positive, and so on...

How do I stop the calcuation from performing every time a new number is entered somewhere on the form?

Thanks for any insight!