#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vs = vector<string>;
using P = pair<int, int>;
using T = tuple<int, int, int>;
using vp = vector<P>;
using vt = vector<T>;
const int mod = 1000000007;
const double EPS = 1e-9;
const int INF = (1 << 30) - 1;
const ll LINF = (1LL << 62) - 1;
inline int in() {
  int x;
  scanf("%d", &x);
  return x;
}
inline ll lin() {
  ll x;
  scanf("%lld", &x);
  return x;
}
inline char chin() {
  char x;
  scanf("%c", &x);
  return x;
}
inline string stin() {
  string x;
  cin >> x;
  return x;
}
inline double din() {
  double x;
  scanf("%lf", &x);
  return x;
}
template <typename T>
inline ll suma(const vector<T>& a) {
  ll res(0);
  for (auto&& x : a) res += x;
  return res;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
char itoa(int n) { return n + '0'; }
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
int main() {
  int r = in();
  double ans = 2 * M_PI * r;
  cout << fixed << setprecision(10) << ans << endl;
  return 0;
}
