#include <bits/stdc++.h>
using namespace std;
template <typename T1, typename T2>
inline void chmin(T1 &a, T2 b) {
  if (a > b) a = b;
}
template <typename T1, typename T2>
inline void chmax(T1 &a, T2 b) {
  if (a < b) a = b;
}
const long long LLINF = (1LL << 60);
const long long INF = (1LL << 30);
const double DINF = std::numeric_limits<double>::infinity();
const long long MOD = 1000000007;
const long long MAX_N = 100010;
signed main() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(false);
  double R;
  cin >> R;
  cout << R * 2.0 * 3.1415926535 << endl;
  return 0;
}
