void A()
{
   TGraphAsymmErrors graph2("AA.txt");

   graph2.SetTitle(" Charged Multiplicity");
   graph2.SetMarkerStyle(kOpenCircle);
   graph2.SetMarkerColor(kBlue);
   graph2.SetLineColor(kBlue);
   TCanvas* mycanvas = new TCanvas();
   graph2.DrawClone("APE");

   TF1 f("f","[0]*([1]*TMath::Exp(-[1]*x) * TMath::Exp(-[2]*TMath::Exp(-[1]*x))) * (1 + [2]*(1-TMath::Exp(-[1]*x)))",10,30);


   f.SetParNames("c","b","eta");
   f.SetParameter(0, 0.2); // c (normalization constant)
   f.SetParameter(1, 0.2); // b
   f.SetParameter(2, 4); // eta

   graph2.Fit(&f, "ME");

   f.DrawClone("Same");

}