I have three columns: "Number 1," "Number 2," and "Difference" (which
is Num 1 - Num2).

In a given row, I want to be able to enter any 2 of the fields and have
Excel calculate the 3rd one.

I.e.: (=x are the numbers excel calculates. The other numbers I type
in)
Num1 Num2 Diff
4 2 =2
=2 1 1
3 =2 1


How can this be done?

Thanks,
Jordan