#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
using Edge = pair<ll, ll>;
using Graph = vector<vector<Edge>>;
const int mod = 1000000007;
struct __INIT {
  __INIT() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
  }
} __init;
struct HashPair {
  template <class T1, class T2>
  size_t operator()(const pair<T1, T2>& p) const {
    auto hash1 = hash<T1>{}(p.first);
    auto hash2 = hash<T2>{}(p.second);
    size_t seed = 0;
    seed ^= hash1 + 0x9e3779b9 + (seed << 6) + (seed >> 2);
    seed ^= hash2 + 0x9e3779b9 + (seed << 6) + (seed >> 2);
    return seed;
  }
};
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
const long long INF = 1LL << 60;
template <typename TYPE, size_t SIZE>
size_t array_length(const TYPE (&)[SIZE]) {
  return SIZE;
}
struct mint {
  ll x;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint& operator+=(const mint a) {
    if ((x += a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator-=(const mint a) {
    if ((x += mod - a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    if (!t) return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1) a *= *this;
    return a;
  }
  mint inv() const { return pow(mod - 2); }
  mint& operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};
struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i) fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i) ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n) return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};
ll getDigit(ll n) {
  ll cnt = 0;
  while (n != 0) {
    n /= 10;
    cnt++;
  }
  return cnt;
}
bool isPrime(ll n) {
  if (n == 1) return false;
  for (ll i = 2; i * i <= n; i++) {
    if (n % i == 0) return false;
  }
  return true;
}
struct Sieve {
  ll n;
  vector<ll> f, primes;
  Sieve(ll n = 1) : n(n), f(n + 1) {
    f[0] = f[1] = -1;
    for (ll i = 2; i <= n; ++i) {
      if (f[i]) continue;
      primes.push_back(i);
      f[i] = i;
      for (ll j = i * i; j <= n; j += i) {
        if (!f[j]) f[j] = i;
      }
    }
  }
  bool isPrime(ll x) { return f[x] == x; }
  vector<ll> factorList(ll x) {
    vector<ll> res;
    while (x != 1) {
      res.push_back(f[x]);
      x /= f[x];
    }
    return res;
  }
  vector<P> factor(ll x) {
    vector<ll> fl = factorList(x);
    if (fl.size() == 0) return {};
    vector<P> res(1, P(fl[0], 0));
    for (ll p : fl) {
      if (res.back().first == p) {
        res.back().second++;
      } else {
        res.emplace_back(p, 1);
      }
    }
    return res;
  }
};
ll gcd(ll a, ll b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
ll lcm(ll a, ll b) { return a * b / gcd(a, b); }
struct binary_compare {
  bool operator()(const pair<ll, ll>& value, const ll& key) {
    return (value.first < key);
  }
  bool operator()(const ll& key, const pair<ll, ll>& value) {
    return (key < value.first);
  }
};
bool PSecComp(const pair<ll, ll>& a, const pair<ll, ll>& b) {
  if (a.second == b.second)
    return a.first > b.first;
  else {
    return a.second > b.second;
  }
}
int main() {
  ll r;
  cin >> r;
  cout << fixed << setprecision(20) << 2 * M_PI * r << endl;
}
