Worksheet Event Change only running coding once

Hello All,

It's been a long time since i have really done any coding and i'm having some issues. Currently i'm trying to write an worksheet event change macro. The situation is this, the worksheet has several columns, The key columns here is A and F. Now what i'm trying to do is that after a person inputs a value into column F and either clicks away, tabs over, or hits enter, it will check the corresponding column A cell as the entered information in F was. If A is blank then it pops up a question box. If A isn't blank then nothing happens.

Dim R As Range
Dim Cell As Range




Set R = Intersect(Target, Range("F:F"))


    If Not R Is Nothing Then
            For Each Cell In R
                If Cell.Value <> "" Then
                    If MsgBox("Is this a Public Tract/Parcel Project?", vbQuestion + vbYesNo, "Public or Private") = vbNo Then
                        Cells(Target.Row, "A").Value = "PR"
                    Else
                        Cells(Target.Row, "A").Value = "PUB"
                    End If
                End If
            Next Cell
    End If

Now I have been researching and trying my best to make it so the question only appears after a value is entered in to "F" and then it doesn't appear again, unless say the information from "F" is deleted. So basically if "F" is blank, it runs a question to fill in "A". Then as long as the value of "F" isn't deleted, then question doesn't come up again, unless the value of "F" is deleted and "F" is now blank.. Now note the value in "F" is going to always be a string of numbers like "6730". 4 to 5 digits in length.