#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	const double PI  =3.141592653589793238463;
	double R;
	cin >> R;
	cout << 2 * PI * R << '\n';
	return 0;
}