#include <stdio.h>
#include <time.h>
#include <math.h>
int main()
{ long int i, n=1000000000;
  long t1, t2;
  double pi=0.0, x, use;
  t1 = clock();
  for (i=1; i<=n; i++)
    { x  = i;
      pi = pi + 6.0/(x*x); }
  t2 = clock();
  use = (double)(t2 - t1)/CLOCKS_PER_SEC;
  pi = sqrt(pi);
  printf("n=%ld, pi=%f\n",n,pi);
  printf(" use time=%f  sec\n",use);
  return 0; }