void linearFit(int N, double x[],double y[], double par[]) {

  double sumx = 0., sumx2 = 0., sumy = 0., sumxy = 0.;

  for (int i = 0; i < N; i++) {
    sumx  += x[i];
    sumx2 += x[i]*x[i];
    sumy  += y[i];
    sumxy += x[i]*y[i];
  }

  double delta = N*sumx2 - sumx*sumx;
  par[0] = (sumx2*sumy - sumx*sumxy)/delta;
  par[1] = (N*sumxy - sumx*sumy)/delta;
  
}