#include <bits/stdc++.h>
using namespace std;
template <typename Val>
Val gcd(Val a, Val b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
template <typename Val>
Val vec_max(vector<Val> v) {
  return *max_element(v.begin(), v.end());
}
template <typename Val>
Val vec_min(vector<Val> v) {
  return *min_element(v.begin(), v.end());
}
int main() {
  double N;
  cin >> N;
  double pi = 3.14159265358979323846;
  N = 2 * N * pi;
  cout << fixed << setprecision(4) << N << endl;
}
