Compile error Variable not defined...?

I am new to VBA and I am trying to follow this tutorial (see link)

http://www.homeandlearn.org/a_string_exercise.html

I entered the code as such...

Sub changeblock()

Dim productcode As String
Dim letters As String
Dim numbers As String
Dim newcode As String

productcode = "PD-23-23-45"
letters = Left(productcode, 2) & c
numbers = Mid(productcode, 3)
newcode = letters & numbers
MsgBox newcode

End Sub


I get the compile error variable not defined message.

What the heck is going on here?

-Thanks