If text1.text <> "" then
    If letra1 = "E" Then
        label1.Caption = letra1
        Text1.Text = ""
    Else
        MsgBox "Letra Incorrecta"
        List1.AddItem (letra1)
        Text1.Text = ""
    End If
end if
If text2 <> "" then
    If letra2 = "S" Then
        label2.Caption = letra2
        Text1.Text = ""
    Else
        MsgBox "Letra Incorrecta"
        List1.AddItem (letra2)
        Text1.Text = ""
    End If
end if