#include <bits/stdc++.h>
using namespace std;
template <typename T>
void print(const T& v) {
  cerr << v << ' ';
}
template <typename T1, typename... T2>
void print(const T1& first, const T2&... rest) {
  print(first);
  print(rest...);
}
typedef priority_queue<pair<int, int>, vector<pair<int, int> >,
                       greater<pair<int, int> > >
    pq;
template <typename T1, typename T2>
T1 setbit(T1 N, T2 pos) {
  return N = N | ((T1)1 << pos);
}
template <typename T1, typename T2>
T1 resetbit(T1 N, T2 pos) {
  return N = N & ~((T1)1 << pos);
}
template <typename T1, typename T2>
bool checkbit(T1 N, T2 pos) {
  return (bool)(N & ((T1)1 << pos));
}
int nextN(int N) { return N + (N & (-N)); }
int parentN(int N) { return N - (N & (-N)); }
template <typename T>
T extGcd(T a, T b, T& x, T& y) {
  if (a == 0) {
    x = 0, y = 1;
    return b;
  }
  T x1, y1, gcd = extGcd(b % a, a, &x1, &y1);
  x = y1 - (b / a) * x1, y = x1;
  return gcd;
}
template <typename T>
T bigMod(T a, T p, T m) {
  if (p == 1) return a;
  T x = bigMod(a, p / 2, m) % m;
  x = (x * x) % m;
  return (p & 1) ? (x * a) % m : x;
}
template <typename T>
T modMul(T a, T b, T m) {
  return ((a % m) * (b % m)) % m;
}
template <typename T>
T modAdd(T a, T b, T m) {
  return ((a % m) + (b % m)) % m;
}
template <typename T>
T modSub(T a, T b, T m) {
  return (a - b + m) % m;
}
template <typename T>
T modInverse(T a, T m) {
  return bigMod(a, m - 2, m);
}
const int maxn = 1e5 + 5;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int r;
  cin >> r;
  double x = 2 * acos(-1.0) * r;
  cout << setprecision(10) << fixed << x;
  return 0;
}
