#include <iostream>

void CountTotalInputParticles() {

  file = new TFile("input.root", "READ");
  tree = (TTree *) file->Get("gRooTracker");
  
  int brStdHepN;
  tree->SetBranchAddress("StdHepN",         &brStdHepN);

  int n_total_input_particles = 0;
  for (int iEntry = 0; iEntry < tree->GetEntries(); ++iEntry) {
    tree->GetEntry(iEntry);
    n_total_input_particles += brStdHepN;
  }

  std::cout << "Total Input Particles = " << n_total_input_particles << std::endl;
}