#include <bits/stdc++.h>
using namespace std;
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;
}
const long long INF = 100100100100100;
const int MOD = 1000000007;
long long mod_pow(long long N, long long M) {
  if (M == 0) return 1;
  long long res = mod_pow((N * N) % MOD, M / 2);
  if (M & 1) res = (res * N) % MOD;
  return res %= MOD;
}
int main() {
  double pi = 3.14159265;
  double N;
  cin >> N;
  cout << N * 2 * pi << endl;
}
