#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll INF = numeric_limits<ll>::max();
const double PI = 3.14159265;
int main() {
  double R;
  cin >> R;
  cout << fixed << setprecision(10);
  cout << 2 * PI * R << endl;
  return 0;
}
