#include<algorithm>
#include<bitset>
#include<cmath>
#include<cstdlib>
#include<iomanip>
#include<iostream>
#include<map>
#include<queue>
#include<set>
#include<string>
#include<utility>
#include<vector>

using namespace std;
 
typedef long long ll;

#define PB push_back
#define MP make_pair
#define F first
#define S second

template<typename T,typename U>
bool chmax(T& max, const U& x){
	if(max<x){
		max = x;
		return true;
	}
	return false;
}

template<typename T,typename U>
bool chmin(T& min, const U& x){
	if(min>x){
		min = x;
		return true;
	}
	return false;
}


int main(){
	double r;
  	cin >> r;
  	cout << 2*r*M_PI << endl;
  	cout << fixed << setprecision(8);
}