#include <bits/stdc++.h>
using namespace std;
template <typename T>
inline T abs(T x) {
  return x > 0 ? x : -x;
}
template <typename T>
inline T sqr(T x) {
  return x * x;
}
template <typename T>
inline T max(T a, T b, T c) {
  return max(max(a, b), c);
}
template <typename T>
inline T min(T a, T b, T c) {
  return min(min(a, b), c);
}
template <typename T>
inline bool toMax(T& a, T b) {
  return a < b ? a = b, 1 : 0;
}
template <typename T>
inline bool toMin(T& a, T b) {
  return a > b ? a = b, 1 : 0;
}
template <typename T>
inline T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
template <typename T, typename T1>
inline T qpower(T x, T p, T1 mod) {
  T re = 1;
  x %= mod;
  while (p) {
    if (p & 1) re = (re * x) % mod;
    x = (x * x) % mod;
    p >>= 1;
  }
  return re;
}
template <typename T1, typename T2>
inline T1 power(T1 x, T2 p) {
  return x < 0 && p < 1 && -1 < p ? p != 0 ? -pow(-x, p) : 1 : pow(x, p);
}
inline bool mr(long long x, long long b) {
  long long k = x - 1;
  while (k) {
    long long cur = qpower(b, k, x);
    if (cur != 1 && cur != x - 1) return false;
    if ((k & 1) == 1 || cur == x - 1) return true;
    k >>= 1;
  }
  return true;
}
inline bool isprime(long long x) {
  if (x == 46856248255981ll || x < 2) return false;
  if (x == 2 || x == 3 || x == 7 || x == 61 || x == 24251) return true;
  return mr(x, 2) && mr(x, 61);
}
template <typename T>
inline void read(T& res) {
  char c;
  res = 0;
  bool f = 0;
  while ((c = getchar()) < 48)
    if (c == '-') f = 1;
  do res = (res << 1) + (res << 3) + (c ^ 48);
  while ((c = getchar()) > 47);
  if (f) res = -res;
}
template <typename T>
inline void write(T x) {
  if (x < 0) putchar('-'), x = -x;
  if (x > 9) write(x / 10);
  putchar(x % 10 ^ 48);
}
template <typename T>
inline string itos(T x) {
  stringstream ss;
  string str;
  ss << x;
  ss >> str;
  return str;
}
const double PI = acos(-1);
const double e = 2.718281828459;
const long long MOD = 1000000007;
const int MAXN = 50005;
const int MAX = 0x3f3f3f3f;
const int INF = 0x80000000;
int main() {
  double r;
  cin >> r;
  cout << 2 * PI * r << endl;
  return 0;
}
