#include <bits/stdc++.h>
#define Pi 3.141592653589793
using namespace std;

int main(int argc, char const *argv[])
{
    int n;
    cin >> n;
    int t = 2 * n;
    double r = Pi * t;
    // printf("%lf\n", r);
    cout << fixed << setprecision(20) << r << endl;

    return 0;
}
