#include <bits/stdc++.h>
using namespace std;
template <typename T>
inline T S(T a) {
  return a * a;
}
template <class T>
inline string tostring(T a) {
  ostringstream os("");
  os << a;
  return os.str();
}
template <typename T>
inline long long tolong(T a) {
  long long res;
  istringstream os(a);
  os >> res;
  return res;
}
template <typename T>
inline T gcd(T a, T b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
template <typename T>
inline T bigmod(T a, T b, T m) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0)
    return S(bigmod(a, b / 2, m)) % m;
  else
    return (a % m * bigmod(a, b - 1, m)) % m;
}
bool prime[20000009];
vector<long long> prm;
void seive() {
  memset(prime, true, sizeof(prime));
  for (long long i = 2; i * i <= 20000009; i++) {
    if (prime[i] == true) {
      prm.push_back(i);
      for (long long j = i * i; j <= 20000009; j += i) prime[j] = false;
    }
  }
}
void pr(int f) {
  if (f == 1)
    cout << "YES\n";
  else
    cout << "NO\n";
}
map<int, int> mp;
set<int> s;
int main() {
  ios_base::sync_with_stdio(0);
  double x;
  cin >> x;
  x = (x * 3.141592653589793 * 2.0);
  cout << x << endl;
}
