void tutorial9b(){
	
	TFile *input = new TFile("tutorial9.root","read");
	
	TTree *tree = (TTree*)input->Get("tree");
	
	double pT[4], eta[4], phi[4], eng[4];
	
	TLorentzVector M1(0,0,0,0);
	TLorentzVector M2(0,0,0,0);
	
	tree->SetBranchAddress("eta", &eta);
	tree->SetBranchAddress("pT" , &pT );
	tree->SetBranchAddress("phi", &phi);
	tree->SetBranchAddress("eng", &eng);
	
	
	TH1F *Zmass = new TH1F("Zmass", "Zmass; Mass (Gev/c^2) ; Events" ,100 , 80 , 100);
	
	int nentries = tree->GetEntries();
	
	//for (int i = 0 ; i < nentries; i++)
	//{
		
	//tree->GetEntry(i);
		
	for(int m = 0 ; m < nentries ; m++)
		{

		if (fabs(pT[m]>142)) continue;       
		//if (phi[m]>-10.4) continue; 
		//if (eta[m]>-3.2) continue;
		
		if (m==0) {M1.SetPtEtaPhiE(fabs (pT[m]), eta[m], phi[m], eng[m]);}
		if (m==1) {M2.SetPtEtaPhiE(fabs (pT[m]), eta[m], phi[m], eng[m]);}
		
		double ZM = (M1+M2).M();
		
		Zmass->Fill(ZM);
		}
	//}
	
	TCanvas *c = new TCanvas();
	Zmass->Draw();

}