Private Function ToCharStr(num As Integer) As String
    Dim chars As String = "abcdefghijklmnopqrstuvwxyz"
 
    num = num - 1
 
    Dim res As String = ""
 
    For i As Integer = 1 To 3
        res = chars(num Mod 26) + res
        num = num \ 26
    Next
 
    Return res
End Function

Private Function CharStrToInt(str As String) As Integer
    Dim chars As String = "abcdefghijklmnopqrstuvwxyz"
 
    Dim l As Integer = str.Length
 
    Dim val As Integer = 0
 
    For i As Integer = 0 To l - 1
        val *= 26
        val += chars.IndexOf(str(i))
    Next
 
    Return val + 1
End Function