#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <map>
#include <iomanip>
#include <cmath>
#include <climits>

using namespace std;
using ll = long long int;
const long double PI = 3.1415926535;
int main () {
 ll r;
 cin >> r;
 cout << fixed << setprecision(10) << (r * 2 * PI) << endl;
}