public static double SimpleMonteCarlo2(PayOff payoff, double expiry, double spot, double vol, double r, ulong number_of_paths) { double variance = vol * vol * expiry; double root_variance = Math.Sqrt(variance); double moved_spot = spot * Math.Exp(r * expiry - 0.5 * variance); double this_spot; double running_sum = 0; for (ulong i = 0; i < number_of_paths; i++) { double this_gaussian = MyRandom.GetOneGaussianByBoxMuller(); this_spot = moved_spot * Math.Exp(root_variance * this_gaussian); running_sum += payoff.Do(this_spot); } return Math.Exp(-r * expiry) * (running_sum / number_of_paths); }