Найти произведение ряда (цикл For) - VB

Imports System.Math
Public Class Form3
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Me.Close()
    End Sub
 
    Private Sub Form3_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        ListBox1.Visible = False
        ListBox1.BackColor = Color.Blue
        ListBox1.ForeColor = Color.Coral
        ProgressBar1.Visible = False
    End Sub
 
    Private Sub Выполнить_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Выполнить.Click
        Dim a As Double : Dim x As Double : Dim n As Integer
        Dim k As Double : Dim y As Double : Dim Dialog As New ColorDialog
        ProgressBar1.Visible = True
        ProgressBar1.Value = 0
        ListBox1.Visible = True
        ListBox1.Items.Clear()
        a = Val(Поле_А.Text)
        x = Val(Поле_Х.Text)
        n = Val(Поле_N.Text)
        ProgressBar1.Maximum = n
        For k = 1 To n
            y = Cos(a - x * k)
            ListBox1.Items.Add("№" & k & "  Y =" & y)
            ProgressBar1.Value = ProgressBar1.Value + 1
        Next
        MsgBox(ProgressBar1.Value)
        If Заливка_фона.Checked And Цвет_фона.Checked Then
            Dialog.Color = ListBox1.BackColor
            Dialog.ShowDialog()
            ListBox1.BackColor = Dialog.Color
            Dialog.Color = ListBox1.ForeColor
            Dialog.ShowDialog()
            ListBox1.ForeColor = Dialog.Color
        ElseIf Заливка_фона.Checked Then
            Dialog.Color = ListBox1.BackColor
            Dialog.ShowDialog()
            ListBox1.BackColor = Dialog.Color
        ElseIf Цвет_фона.Checked Then
            Dialog.Color = ListBox1.ForeColor
            Dialog.ShowDialog()
            ListBox1.ForeColor = Dialog.Color
        End If
    End Sub
End Class