Calculate pi in OpenMP-parallel

/*
 * Calculate $\pi = 4 \int_0^1 \sqrt{1 - x^2} \, dx$
 * using a rectangle rule $\sum_{i=1}^N f(x_0 + i h - h/2) h$
 *
 * Compile as   $> gcc -Wall -Wextra -fopenmp -lm
 * Run as       $> OMP_NUM_THREADS=2 ./a.out
 */
#include <stdio.h>
#include <math.h>
#include <omp.h>

int main()
{
    const int N = 10000000;
    const double L = 1.0;
    const double h = L / N;
    const double x_0 = 0.0;

    double pi;
    double t_1, t_2;

    int i;
    double sum = 0.0;

    t_1 = omp_get_wtime();

#pragma omp parallel for reduction(+: sum) schedule(static)
    for (i = 0; i < N; ++i)
    {
        double x = x_0 + i * h + h/2;
        sum += sqrt(1 - x*x);
    }

    t_2 = omp_get_wtime();

    pi = sum * h * 4.0;

    printf("omp_get_max_threads(): %d\n", omp_get_max_threads());
    printf("time: %f\n", t_2 - t_1);
    printf("pi ~ %f\n", pi);

    return 0;
}