VB里,怎么把常数2定义成双精度?

下面是我编的程序,运行了,只算n=50的时候,n=1000就不算了,究竟怎么把常数2定义成双精度呢?

Dim yuanzhoulv As Double, n As Double

Private Sub Command1_Click()
yuanzhoulv = 2
For n = 1 To 2000
yuanzhoulv = yuanzhoulv * ((2 * n) ^ 2) / ((2 * n - 1) * (2 * n + 1))
If n = 50 Then
Text1.Text = yuanzhoulv
Else
If n = 1000 Then
Text1.Text = Text1.Text & Chr(13) + Chr(10) & yuanzoulv
End If
End If
Next n

End Sub