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

#define rep(i, n) for (int i = 0; i < (int)(n); i++)


int main() {
	const double PI = 3.14159265358979323846;
	double R;
	cin >> R;
	cout << fixed << setprecision(8) << R * 2 * PI << endl;
}
