#include <bits/stdc++.h>
using namespace std;
signed _main();
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  _main();
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
using v = vector<T>;
template <class T>
using vv = vector<v<T>>;
const long long MOD = 1e9 + 7;
const long long INF = 1LL << 60;
signed _main() {
  long double R;
  cin >> R;
  cout << 2.0 * M_PI * R << endl;
  return 0;
}
