Adobe form field with Javascript changing text when tab or click out

I successfully entered Javascript to calculate a participant study ID from another number using <event.value = Math.pow((this.getField("participantID").value), (1/2.71828))>. When I enter a number into the field, it automatically calculates a participant ID. But when I hit Tab to move on, or click on another field in the form, the number changes. I can't figure out what is causing this and how to keep the original calculation.

Thanks for any help!