double myFunc(double *x, double *par){
  double result = 
    par[0]*exp(-((x[0]-par[1])/(2*par[2]))*((x[0]-par[1])/(2*par[2]))) + par[3];
  return result;
}

void fit() {

  TH1F histo("pippo","Segnale con Fondo",100,0.,1.);

  int num= 10000;
  for (int i = 0; i<num;++i) {
    float rnd=gRandom->Gaus(0.5,0.05);
    histo.Fill(rnd);
  }

  int numFondo= 10000;
  for (int i = 0; i<numFondo;++i) {
    float rnd=gRandom->Rndm();
    histo.Fill(rnd);
  }

  gStyle->SetOptStat(111111);
  gStyle->SetOptFit(111111);
  histo.SetMinimum(0);

  TF1 miaFunc("miaFunc",myFunc,0.,1.,4);
  miaFunc.SetParLimits(2,0,100);
  // miaFunc.SetParameter(0,100);
  miaFunc.SetParameter(1,0.5);
  miaFunc.SetParameter(2,0.5);
  // miaFunc.SetParameter(3,10);

  histo.Fit("miaFunc");
  histo.DrawCopy();

}