#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
using PP = tuple<ll, ll, ll>;
using S = multiset<ll>;
using vi = vector<ll>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
const ll inf = 1001001001001001;
const int INF = 1001001001;
const int mod = 1000000007;
const double eps = 1e-10;
template <class T>
bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
void out(T a) {
  cout << a << '\n';
}
template <class T>
void outp(T a) {
  cout << '(' << a.first << ',' << a.second << ')' << '\n';
}
template <class T>
void outvp(T v) {
  for (long long i = 0; i < (long long)(v.size()); i++)
    cout << '(' << v[i].first << ',' << v[i].second << ')';
  cout << '\n';
}
template <class T>
void outv(T v) {
  for (long long i = 0; i < (long long)(v.size()); i++) {
    if (i) cout << ' ';
    cout << v[i];
  }
  cout << '\n';
}
template <class T>
void outvv(T v) {
  for (long long i = 0; i < (long long)(v.size()); i++) outv(v[i]);
}
template <class T>
bool isin(T x, T l, T r) {
  return (l) <= (x) && (x) <= (r);
}
template <class T>
void YesNo(T b) {
  if (b)
    out("Yes");
  else
    out("No");
}
void decimal(int x) { cout << fixed << setprecision(x); }
ll GCD(ll a, ll b) {
  if (b == 0) return a;
  return GCD(b, a % b);
}
ll LCM(ll a, ll b) { return (a / GCD(a, b)) * (b / GCD(a, b)) * GCD(a, b); }
ll POW(ll a, ll b) {
  a %= mod;
  if (b == 0) return 1;
  if (b & 1) return a * POW(a, b - 1) % mod;
  ll k = POW(a, b / 2);
  return k * k % mod;
}
vi calc(ll x) {
  vi res;
  while (x > 0) {
    res.emplace_back(x % 10);
    x /= 10;
  }
  reverse(res.begin(), res.end());
  return res;
}
void solve() {
  int r;
  cin >> r;
  double ans = 2 * M_PI * r;
  decimal(10);
  out(ans);
}
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
}
