#include <bits/stdc++.h>
using namespace std;
using ll = long long;

ll solve() {
    ll R;
    cin >> R;
    cout << R * M_PI * 2 << "\n";
    return 0;
}

int main() {
    cout.setf(ios::fixed);
    cout.precision(16);
    solve();
    return 0;
}