HowToReadTree()
{

  // declare  tree
  TChain chain("miniTree");

  // chain all input files
  chain.Add("../data/analCutFlow.egstream.00179710_25July.root");
  chain.Add("../data/analCutFlow.egstream.00179710_25July.root");
  chain.Add("../data/analCutFlow.egstream.00179710_25July.root");

  // set address to your variables
  Int_t  RunNumber;
  chain.SetBranchAddress("RunNumber", &RunNumber);

  // create histograms
  TH1F *histRunNumber = new TH1F("histRunNumber","Run number",20,179700,179720);

  // loop over all entries in the chain
  Int_t nevent = chain.GetEntries(); 
  for (Int_t i=0;i<nevent;i++) {
    chain.GetEvent(i);  
    std::cout << RunNumber << std::endl;
    //fill histogram
    histRunNumber->Fill(RunNumber);
  }
  
  // draw your histogram
  histRunNumber->Draw();
  
  // open file to store your histogram
  TFile  outputFile("histos.root","RECREATE");
  outputFile.cd();
  histRunNumber->Write();
  outputFile.Close();

}