Function BlackScholes(Underlying, Strike, RiskFree, expTime, Volatility)
    d1 = (Log(Underlying / Strike) + RiskFree * expTime) / (Volatility * Sqr(expTime)) + _
    0.5 * Volatility * Sqr(expTime)
    BlackScholes = Underlying * Application.NormSDist(d1) - Strike * Exp(-expTime * RiskFree) * _
    Application.NormSDist(d1 - Volatility * Sqr(expTime))
End Function