Private Sub CheckInput(ByVal input As String)
        If String.IsNullOrEmpty(input) Then
            Throw New ArgumentNullException("input")
        End If
 
        For Each c As Char In input
            If Not Char.IsLetter(c) Then
                Throw New ArgumentException("Invalid inputstring")
            End If
        Next
    End Sub
 
    Private Function StringToNumber(ByVal input As String) As Integer
        CheckInput(input)
        input = input.ToUpper()
 
        Dim result As Integer = 0
 
        For index As Integer = input.Length - 1 To 0 Step -1
            Dim num As Integer = AscW(input(index)) - 64
            Dim pos As Integer = input.Length - index - 1
            result += num * (26 ^ pos)
        Next
 
        Return result
    End Function
 
    Private Function NumberToString(ByVal input As Integer) As String
        Dim result As String = String.Empty
        Dim rest As Integer = 0
 
        If input > 26 Then
            Do
                input = Math.DivRem(input, 26, rest)
                If rest = 0 Then
                    input -= 1
                    rest = 26
                End If
 
                result = ChrW(rest + 64) & result
            Loop While input > 26
        End If
 
        result = ChrW(input + 64) & result
 
        Return result
    End Function
 
    Private Function Increment(ByVal input As String, ByVal count As Integer) _
      As String
        Dim value As Integer
 
        If Integer.TryParse(input, value) Then
            Return (value + 1).ToString
        Else
            Return NumberToString(StringToNumber(input) + count)
        End If
    End Function
 
    Private Function Decrement(ByVal input As String, ByVal count As Integer) _
      As String
        Dim value As Integer
 
        If Integer.TryParse(input, value) Then
            Return (value - 1).ToString
        Else
            Return NumberToString(StringToNumber(input) - count)
        End If
    End Function