#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
using namespace std;

typedef long long ll;

const int INF = 1e9;

int main() {
    double R;
    cin >> R;

    cout << fixed << setprecision(6) << 2.0*R*M_PI << "\n";
}