I have the following Data
Col A Col B Col C
2 3 =A1-B1

The user enters data in Cell A1 Or Cell B1 and Cell C1 Calculates the results

I want the following
If the user enters any value in Cell A1 Or Cell B1 and the Value computed in
Cell C1 returns to Negative . he gets a pop up saying "Invalid Range"

Eg in the above Cell C Will Return -1 in such case I want to get a pop up In
valid Range
If Cell C1 Is Positive no message should be displayed

I tried Data Valadation but It does not work with Formulas . . . Or Does it
work with Formula which I am not aware of ?

Thank You