Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles Button1.Click
        Dim a As Double = CInt(TextBox1.Text)
        Dim b As Double = CInt("1")
 
        If Button1.Text = "OFF" Then
            Button1.Text = "ON"
            TextBox1.Text = a + b
            Exit Sub
        End If
 
        If Button1.Text = "ON" Then
            Button1.Text = "OFF"
            TextBox1.Text = a - b
            Exit Sub
        End If
    End Sub