#include<iostream>

double piquart ( double incert) {
  int cpt=3;
  bool pair = false;
  double pisurquatre=1;
  while ( 1/(double)cpt> incert) {
    pisurquatre = pair ? pisurquatre+1 /(double)cpt : pisurquatre-1/(double)cpt;
    cpt+=2;
    pair = pair ? false : true;
  }
  return pisurquatre;
}

int main () {
  double precision;
  std::cout<< " Entrez la precision ( < 1 ) a laquelle vous voulez apprecier la valeur de pi" << std::endl;
  std::cin>>precision;
  std::cout<< 4*piquart(precision)<<std::endl;
  return 0;
}