void get_FRAOrder() {

  TFile*        fHitsFile    = TFile::Open("Gem_3Stations_15GeV_n6000_hitsSELECTOR.root");
  TTree*        fHitsTree    = (TTree*) fHitsFile->Get("cbmsim");

  FairEventHeader* fEHeader  = new FairEventHeader();

  fHitsTree->SetBranchAddress("EventHeader.",&fEHeader);

  Int_t nofEvents = fHitsTree->GetEntriesFast();

  TH1F* entriesOrder = new TH1F("entriesOrder","entries order",nofEvents,-0.5,nofEvents-0.5);

  for ( Int_t iEvent = 0 ; iEvent < nofEvents ; iEvent++ ) {
    fHitsTree->GetEntry(iEvent);
    entriesOrder->Fill(iEvent,fEHeader->GetMCEntryNumber());
  }

  entriesOrder->Draw();
}