#include <stdio.h>
#include <stdlib.h>
#include <omp.h>

int main() {
  long long int i, n=10000000000;
  double start_time, end_time;
  double x, pi;
  double sum = 0.0;
  double step = 1.0/(double) n; 

  start_time = omp_get_wtime();
#pragma omp parallel for default(shared) private(i,x)
    for (i = 0; i < n; i++) {
      x = (i+0.5)*step;
#pragma omp atomic
      sum +=  4.0/(1.0+x*x);
    }
  pi = step * sum;
  end_time = omp_get_wtime();
  
  printf("pi = %17.15f\n",pi);
  printf("time to compute = %g seconds\n", (double)(end_time - start_time));
  return 0;
}