#include <TH1.h>
#include <TStyle.h>
#include <TCanvas.h>
#include <iostream>
#include <TMath.h>
#include <TFile.h>
#include <TTree.h>
#include <TStyle.h>
#include <stdlib.h>
#include "TLeaf.h"
#include "TROOT.h"
#include "TFile.h"
#include "TTree.h"
#include "TChain.h"
#include "TNtuple.h"
#include "TBranch.h"
#include "TFileCollection.h"
#include "TGraphErrors.h"

using namespace std;
using namespace TMath;

void test()
{   

  // Creating a new root file to save new histograms
  TFile *histofile = new TFile("NewFile.root","RECREATE");  

  // calling existed files 
  TFile *bkg=new TFile("background.root","READ");  
  TFile *single_top=new TFile("single_top.root","READ");

  // finding the considered histograms in the files      
  TH1 *bkg_lep_PostvdDeltaEta =(TH1*) bkg->Get("lep_PostvdDeltaEta");
  TH1 *singleT_lep_PostvdDeltaEta = (TH1*)single_top->Get("lep_PostvdDeltaEta");

  bkg_lep_PostvdDeltaEta->SetDirectory(histofile); 
  bkg_lep_PostvdDeltaEta->Add(singleT_lep_PostvdDeltaEta, -1);
 
  histofile->Write();
  histofile->Close();
}