/* 求π.cpp */
#include <iostream>
using namespace std;
double arctan(double x);
int main(int argc, char const *argv[]) {
  double a = 16.0 * arctan(1.0 / 5.0);
  double b = 4.0 * arctan(1.0 / 239.0);
  double pi = a - b;
  cout << pi << endl;
  
  return 0;
}
double arctan(double x) {
  double head = x;
  int tail = 1;
  double art = 0;
  while (double(head / tail) > 1e-15) {
    art = (tail % 4 == 1) ? art + head / tail : art - head / tail;
    head *= x * x;
    tail += 2;
    cout << "---------------" << endl;
    cout << tail << endl;
    cout << "---------------" << endl;
  }
  return art;
}