#include <bits/stdc++.h>
using namespace std;
const int INF = ~(1 << 31);
const double EPS = 1e-9;
const double pi = acos(-1);
template <class T>
T smod(T a, T b) {
  return (a % b + b) % b;
}
int main() {
  cin.sync_with_stdio(false);
  double r;
  cin >> r;
  cout << setprecision(15) << fixed << 2 * r * pi << endl;
  return 0;
}
