Macro To Prevent Changing The Value Within A Cell

I would like to write a macro that performs the following function:

I have a value in cell A1

In cell A2, I have the formula "=A1"

I want to make it so that cell A2 can never be directly changed; the only way to change the value in A2 is by changing A1. A2 should always equal A1.

I want a message box to pop up and alert the user whenever they attempt to modify A2, instructing them that if they wish to change A2, they have to change A1.

I had written this macro, and it kinda does the trick:



Private Sub Worksheet_SelectionChange(ByVal Target As Range)


If Range("a2") Range("a1") Then
MsgBox "text goes here"
Range("a2").Value = Range("a1")
End If

End Sub

When I attempt to change cell A2, I get the MsgBox popup and it works how I want it to. However, when I alter cell A1, I get the same popup before cell A2 updates (to be precise, the popup occurs after I click another cell). I would like this to be immediate and without a MsgBox.