void example(){
  using namespace RooFit ;

  gStyle->SetOptStat(0);

  RooRealVar Jpsi_M("Jpsi_M","Jpsi_M",3097,3010,3185);

  RooRealVar sigma_Jpsi_R2("sigma_Jpsi_R2","sigma",14, 7, 20);
  RooRealVar mu_Jpsi_R2("mu_Jpsi_R2","mu",3097, 3050, 3150);
  RooGaussian signal("signal","signal PDF",Jpsi_M,mu_Jpsi_R2,sigma_Jpsi_R2);

  RooRealVar slope("slope","exp slope",-0.005,-0.1,0.1);
  RooExponential exp_pdf("exp_pdf", "exp_pdf", Jpsi_M, slope);

  RooRealVar yield_comb("yield_comb","yield",100,0,1000); //exp
  RooRealVar yield_signal("yield_signal","yield",400,0,2000); //signal

  RooArgList shapes;
  shapes.add(exp_pdf); //combinatorial
  shapes.add(signal); //signal

  RooArgList yields;
  yields.add(yield_comb); //combinatorial
  yields.add(yield_signal); //signal

  //total PDF
  RooAddPdf* total = new RooAddPdf("total","total",shapes,yields);
  RooDataSet* dataset = total -> generate(RooArgSet(Jpsi_M),yield_signal.getValV() + yield_comb.getValV());
  total->fitTo(*dataset, BatchMode(1), PrefitDataFraction(0.1));

  //Plotting
  RooPlot* Jpsi_frame=Jpsi_M.frame(Bins(50),Name("Jpsi mass"),Title(" "));
  dataset->plotOn(Jpsi_frame,MarkerSize(0.9));
  total->plotOn(Jpsi_frame, Components(exp_pdf),FillStyle(1001),VLines(),DrawOption("F"),FillColor(kOrange-4),LineColor(kOrange-4),Name("comb"));
  total->plotOn(Jpsi_frame, Components(signal),LineColor(kRed),Name("signal"));

  total->plotOn(Jpsi_frame);
  // total->paramOn(Jpsi_frame,Layout(0.63,0.93,0.93));
  TLegend* leg = new TLegend(0.17,0.75,0.35,0.9);


  leg->AddEntry(Jpsi_frame->findObject("signal"),"Signal","l");
  leg->AddEntry(Jpsi_frame->findObject("comb"),"Background","l");

  TCanvas* DCanvas = new TCanvas ("DCanvas","Fit",750,750);

  Jpsi_frame->GetYaxis()->SetTitleOffset(0.9);
  Jpsi_frame->GetXaxis()->SetTitle("m(J/#psi(#mu^{+}#mu^{-})) [MeV/c^{2}]");

  Jpsi_frame->Draw();
  leg->Draw();
  DCanvas->SaveAs("fit_example.pdf");

}