#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i=0; i < (int)(n); i++)
using ull = unsigned long long;
using ll = long long;

#define PI 3.14159265359

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

    cout << R * 2 * PI << endl;

    return 0;
}

