#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 5;
const int mod = 1e9 + 7;
double pi = 3.14159265359;
int main() {
  double r;
  cin >> r;
  cout << fixed << setprecision(15) << 2.0 * pi * r << "\n";
}
