#include<iostream>
using namespace std;
int main (void)
{
  int r;
  cin>>r;
  r=r*2*3141592;
  cout<<r/1000000<<"."<<r%1000000;
}
#include <bits/stdc++.h>
using namespace std;

  
int main() {
  const double PI = 3.14159265358979;
  double R;
  cin >> R;
  cout << R*2*PI << endl;
  
  return 0;
  
    }