Помогите разобраться с кодом Do While.. Loop - VB

Private Sub Command1_Click(Index As Integer)
Dim n As Integer, s As String, v As Single
Dim y1 As Variant, y2 As Variant, x1 As Variant, x2 As Variant, a1 As Variant, b1 As Variant
Const a As Variant = 0.01, b As Long = 1000, e As Double = 0.01, pi As Double = 3.141592654
 x1 = a: x2 = b: n = 0
 s = Text1.Text
 v = Val(s)
Do While Abs(x1 - x2) <= 2 * e
 n = n + 1: a1 = x1: b1 = x2
 x1 = (x1 + x2) / 2 - e / 2: x2 = (x1 + x2) / 2 + e / 2
 y1 = pi * x1 ^ 3 + pi * x1 * (3 * v / (pi * x1 ^ 2)) ^ 2 + pi * x1 ^ 2
 y2 = pi * x2 ^ 3 + pi * x2 * (3 * v / (pi * x2 ^ 2)) ^ 2 + pi * x2 ^ 2
 If y1 < y2 Then
 a1 = a1: b1 = x2
 End If
 If y1 = y2 Then
 a1 = x1: b1 = x2
 End If
 If y1 > y2 Then
 a1 = x2: b1 = b1
 End If
Loop
x1 = a1: x2 = b1
MsgBox ответ, vbOKOnly, "Расчитанные параметры"
End Sub