definite integral cos(-x^2) dx from x=-1 to 1

#include <math.h>
#include <stdio.h>
static double f(double x) { return cos(-x * x); }
int main(void) {
  const int n = 10000;
  const double a = -1., b = 1., h = (b - a) / n;
  double s = f(a) + f(b);
  for (int i = 1; i < n; ++i) { s += (i % 2 ? 4 : 2) * f(a + i * h); }
  s *= h / 3;
  printf("%.17f\n", s);
}