#include <bits/stdc++.h>

using namespace std;

#define sz(x) (int) x.size()
#define all(a) a.begin(), a.end()

const double pi = acos(-1.0);


int main() {

    double r;
    cin >> r;

    printf("%.12lf\n", 2 * pi * r);

    return 0;
}
