#include <bits/stdc++.h>

#define ll long long
#define ull unsigned long long

using namespace std;

int main() {
	double R;
	cin >> R;

	double pi = 3.14159265353238;

	double ans = 2.0 * pi * R;
	cout << fixed << setprecision(10) << ans << "\n";


	return 0;
}
