double integrate(int n) {
  double sum, h, x;
  int i;
    h   = 1.0 / (double) n;
    for (i = 0; i <= n; i++) {
      x = h * ((double)i - 0.5);
      sum += 4.0 / (1.0 + x*x);
    }
    return sum * h;
}