Public Function S(ByVal r, ByVal sigma, ByVal S0, ByVal N, ByVal start, ByVal fin)
'Simulate a lognormal dynamic for a stock
Dim stock() As Double
Dim i As Long
Dim dt As Double

ReDim sigma(0 To N)
stock(0) = S0
dt = (fin - start) / N

For i = 1 To N
    temp = alea()
    stock(i) = stock(i - 1) * Exp((r - 0.5 * sigma * sigma) * dt + Sqr(dt) * sigma * Application.WorksheetFunction.NormSInv(temp))
Next i
S = stock
End Function