void sim(Int_t nev=100) {
//  AliLog::SetGlobalLogLevel(AliLog::kError);

  AliSimulation simu;
  simu.SetMakeSDigits("TRD TOF PHOS HMPID EMCAL MUON FMD ZDC PMD T0 VZERO");
  simu.SetMakeDigitsFromHits("ITS TPC");

  simu.SetDefaultStorage("alien://Folder=/alice/simulation/2008/v4-15-Release/Ideal");


  simu.SetSpecificStorage("GRP/GRP/Data",Form("local://%s",gSystem->pwd()));

  TStopwatch timer;
  timer.Start();
  simu.Run(nev);
  WriteXsection();
  timer.Stop();
  timer.Print();
}

WriteXsection()
{
  TPythia6 *pythia = TPythia6::Instance();
  pythia->Pystat(1);
  Double_t xsection = pythia->GetPARI(1);
  Int_t    ntrials  = pythia->GetMSTI(5);

  TFile *file = new TFile("pyxsec.root","recreate");
  TTree   *tree   = new TTree("Xsection","Pythia cross section");
  TBranch *branch = tree->Branch("xsection", &xsection, "X/D");
  TBranch *branch = tree->Branch("ntrials" , &ntrials , "X/i");
  tree->Fill();

  tree->Write();
  file->Close();

  cout << "Pythia cross section: " << xsection 
       << ", number of trials: " << ntrials << endl;
}