#include <stdio.h>
main(void)
{
  long i,n;
  double a=0;
  double sign=1;
  printf("How many iterations?");
  scanf("%d",&n);
  for (i=1; i<n; i++) {
    a += sign / (2*i-1);
    sign = -sign;
  }
  printf("pi is nearly %.10f\n", 4*a);
}