#include <bits/stdc++.h>
using namespace std;
const double pi = 3.1415;
double R;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cin >> R;
  cout << setprecision(6) << fixed << (R * 2.0) * pi << endl;
  return 0;
}
