Imports System.Collections.Generic
Imports System.Console

Module Program
    Sub Main()
        Dim S As String = NextString()

        For i As Integer = Asc("a"c) To Asc("z"c)
            Dim c As Char = Chr(i)
            If Not S.Contains(c) Then
                WriteLine(c)
                Exit Sub
            End If
        Next

        WriteLine("None")
    End Sub

    Function NextString() As String
        Dim result As New List(Of Char)
        Do
            Dim tmp As Integer = Read()
            If tmp < 0 Then Exit Do
            Dim nextChar As Char = Chr(tmp)

            If Not Char.IsWhiteSpace(nextChar) Then
                result.Add(nextChar)
            ElseIf nextChar <> vbCr Then
                Exit Do
            End If
        Loop
        Return New String(result.ToArray())
    End Function

End Module
