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