#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &v : vec) is >> v;
  return is;
}
template <typename T>
bool chmax(T &m, const T q) {
  if (m < q) {
    m = q;
    return true;
  } else
    return false;
}
template <typename T>
bool chmin(T &m, const T q) {
  if (q < m) {
    m = q;
    return true;
  } else
    return false;
}
lint pow_mod(lint base, lint expo) {
  lint ret = 1;
  for (; expo;) {
    if (expo & 1) ret = (ret * base) % 1000000007;
    expo = (expo >> 1);
    base = (base * base) % 1000000007;
  }
  return ret;
}
void solve() {
  float r;
  cin >> r;
  cout << M_PI * 2 * r << endl;
}
int main() {
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
  solve();
  return 0;
}
