Help on calculation between dates (and removing NaN)

[ArtoReinvention]

Hello, I am stuck on something that seems simple, and I've tried navigating the answer here, but still can't get my formula to work.

I have two fields I'm working with, a check-in field, which is a dd/mm/yyyy field, and a check-out which is also a dd/mm/yyyy field.  I don't always use both fields, though - sometimes one is blank, and sometimes both are blank.

I'm trying to calculate the # of nights between the two dates, and the formula is working fine, but returns "NaN" when there is no date.  How can I correct the formula to simply leave that field empty if there are no dates? 

Also, it seems to be wanting to calculate a sum, which I don't want it to do.  Not sure why the Sum and Infinity symbols? 

Here is my formula: 

DATETIME_DIFF({Check-out}, {Check-in}, "days")

Thank you!

[TheTimeSavingCo]

Does this look right?

IF(
  AND(
    {Check-out},
    {Check-in}
  ),
  DATETIME_DIFF({Check-out}, {Check-in}, "days")
)

---
Not sure I'm following about the sum, infinity symbols bit though, could you tell me more about that?

[ArtoReinvention]

YES!  Thank you so much!  That worked.  Appreciate it.