Imports System
 
Module LambertW
	Function LAMBERT(x As Double) As Double
		Dim Wx, Wxcont, L, Lprim, Lsec As Double
		If x < -Math.Exp(-1) Then
			Console.WriteLine("Invalid Argument!")
			Exit Function
		End If
		Wx = 1
		Do Until Math.Abs(Wxcont - Wx) < 0.000000001
			Wxcont = Wx
			L = Wx * Math.Exp(Wx) - x
			Lprim = Math.Exp(Wx) * (Wx + 1)
			Lsec = Math.Exp(Wx) * (Wx + 2)
			Wx = Wx - L / (Lprim - (L * Lsec / (2 * Lprim)))
		Loop
		LAMBERT = Wx
	End Function
	Sub Main()
		Console.WriteLine(LAMBERT(-1.0 / Math.Exp(1.0)))
	End Sub
End Module

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