#include <iostream>
#include <vector>
#include <string>
#include <math.h>
#include <algorithm>
#include <set>
#include <iomanip>
#include <stdio.h>
#include <sstream>
#include <bits/stdc++.h>
using namespace std;
long double pi=3.1415926535897932384626433832795028;
int main(void){
  long double r;
  cin>>r;
  double ans=2*r*pi;
  cout<<fixed<<setprecision(10)<<ans<<endl;
}