How can I populate a field based on another field only on the page load? (Infopath)

Question

I'm creating an Infopath form, and what I'm trying to do is populate an extra textbox with the inital value of the field ONLY while loading.

I know that sounds weird, an example might be better: -Field 1 starts blank, user changes the field to "100" and saves it. Extra_Field stays blank. -When the form opened again, Extra_Field now populates with "100" and user changes Field 1 to "200", Extra_Field stays at "100", saves the form -The field is opened again, Extra_Field now populates "200"...

Any help would be greatly appreciated! Thanks!