Dim creditNum As String
        Dim checkDigit As String
        Dim sumOfDigits As Integer
        Dim num1 As Integer
        Dim num2 As Integer
        Dim num3 As Integer
        Dim num4 As Integer

        creditNum = Me.xNumberTextBox.Text

        ' verify that the credit card number contains five digits
        If creditNum Like "#####" Then
            ' calculate correct check digit
            Integer.TryParse(creditNum.Substring(0, 1), num1)
            Integer.TryParse(creditNum.Substring(1, 1), num2)
            Integer.TryParse(creditNum.Substring(2, 1), num3)
            Integer.TryParse(creditNum.Substring(3, 1), num4)

            'calculate sum of digit

            sumOfDigits = num1 + num2 + num3 + num4

            'calculate check digit
            creditNum = "#####"
            num1 = num1
            num2 = num1 * 2
            num3 = num2
            num4 = num3 * 2
            checkDigit = CStr(sumOfDigits)


            ' determine whether the credit card number ends with the correct check digit
            If creditNum.EndsWith(checkDigit) Then
                ' processed when the credit card is valid
                Me.xMessageLabel.Text = "The credit card number is valid."
            Else
                ' processed when the credit card is not valid
                Me.xMessageLabel.Text = "The credit card number is not valid."
            End If
        Else    ' processed when the user does not enter five digits
            MessageBox.Show("Please enter a five-digit number.", "Georgetown Credit", _
                MessageBoxButtons.OK, MessageBoxIcon.Information)
        End If

        Me.xNumberTextBox.Focus()
    End Sub
End Class