#include <iostream>
#include <iostream>
#include <string>
#include <vector>
#include <stdio.h>
#include <cmath>
#include <algorithm>
#define _USE_MATH_DEFINES
#include <math.h>
using namespace std;


int main() {

	double R;
	cin >> R;
	cout << 2*R * M_PI << endl;



}