TH1F *hist;
float MC_lente(float o, float so, float i, float si, int N)
{
  hist = new TH1F("f","foco",100,0,20);
  
  TRandom *r = new TRandom();
  for(int k = 0; k<N; k++)
  {
    float I = r->Gaus(i,si);
    float O = r->Gaus(o,so);
    float F = I*O/(I+O);
    hist->Fill(F);
  }
  float RMS =hist->GetRMS();
  return RMS; 
}