#include <bits/stdc++.h>

using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    double r;
    cin >> r;
    double pi = atan(1) * 4;
    cerr << pi << endl;
    cout << setprecision(10) << 2 * pi * r << endl;
    return 0;
}
