#include <bits/stdc++.h>
using ull = unsigned long long int;
using ll = long long;
using ld = long double;
using pii = std::pair<int, int>;
using pll = std::pair<ll, ll>;
using vi = std::vector<int>;
using vvi = std::vector<vi>;
using vll = std::vector<ll>;
using vvll = std::vector<vll>;
using vd = std::vector<double>;
using vvd = std::vector<vd>;
using qi = std::queue<int>;
using vpii = std::vector<std::pair<int, int> >;
using vpll = std::vector<pll>;
using namespace std;
const int Mod = (1e9) + 7;
const int INF = 1e9 + 19;
const ll INFL = 1e18 + 19;
const int dx[] = {-1, 0, 0, 1};
const int dy[] = {0, -1, 1, 0};
const int dx2[] = {-1, -1, -1, 0, 0, 0, 1, 1, 1};
const int dy2[] = {1, 0, -1, 1, 0, -1, 1, 0, -1};
template <class T1, class T2>
inline void chmin(T1 &a, T2 b) {
  if (a > b) a = b;
}
template <class T1, class T2>
inline void chmax(T1 &a, T2 b) {
  if (a < b) a = b;
}
template <class T>
inline void pri(T a) {
  cout << a << endl;
}
template <class Z>
using vec = vector<Z>;
template <class T1, class T2>
void update(T1 &a, T2 b) {
  a += b;
  if (a > Mod) a %= Mod;
}
inline void IN(void) { return; }
template <typename First, typename... Rest>
void IN(First &first, Rest &...rest) {
  cin >> first;
  IN(rest...);
  return;
}
inline void OUT(void) {
  cout << "\n";
  return;
}
template <typename First, typename... Rest>
void OUT(First first, Rest... rest) {
  cout << first << " ";
  OUT(rest...);
  return;
}
bool pairsort(pll pl, pll pr) {
  if (pl.first == pr.first) return pl.second > pr.second;
  return pl.first < pr.first;
}
int cntbit(ll a, int n, int j) {
  int res = 0;
  for (int(i) = (j); (i) < (n); (i)++) {
    if (a >> i & 1) {
      res++;
    }
  }
  return res;
}
vector<int> make_bit(int a) {
  vector<int> res;
  for (int i = 0; i < (32); i++)
    if (a & (1 << i)) res.push_back(i);
  return res;
}
bool stdbit(int a, int b) { return a & (1 << b); }
int GCD(int a, int b) {
  if (b > a) return GCD(b, a);
  if (a % b == 0)
    return b;
  else
    return GCD(b, a % b);
}
int LCM(int a, int b) { return a * b / GCD(a, b); }
int roundup(int a, int b) {
  if (a % b == 0)
    return a / b;
  else
    return (a + b) / b;
}
int rounddown(int a, int b) {
  if (a % b == 0)
    return a / b;
  else {
    return (a - b) / b;
  }
}
ll pow(ll a, ll n) {
  ll res = 1;
  while (n > 0) {
    if (n & 1) res *= a;
    a *= a;
    n = n >> 1;
  }
  return res;
}
ll GetDiviserCount(ll N) {
  ll res = 1;
  for (int(i) = (2); (i) < (sqrt(N) + 1); (i)++) {
    ll cnt = 0;
    while (N % i == 0) {
      cnt++;
      N /= i;
    }
    res *= (cnt + 1);
    if (N == 1) break;
  }
  if (N != 1) res *= 2;
  return res;
}
vll GetDivisor(ll N) {
  vll res;
  for (ll i = 1; i * i <= N; i++) {
    if (N % i == 0) {
      res.push_back(i);
      if (i * i != N) res.push_back(N / i);
    }
  }
  sort((res).begin(), (res).end());
  return res;
}
const int max_n = 3 * (1e5) + 1;
const int max_m = 83 * (1e5) + 1;
int n, m, k;
ll N;
int h, w;
vvi tree;
string S;
int a, b, c;
vi v;
int ans;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  double r;
  IN(r);
  double p = acos(-1);
  double ans = 2.0 * r * p;
  cout << fixed << setprecision(15) << ans << endl;
  return 0;
}
