#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vec = vector<ll>;
using vect = vector<double>;
using Graph = vector<vector<ll>>;
ll ctoi(char c) {
  if (c >= '0' && c <= '9') {
    return c - '0';
  }
  return 0;
}
template <typename T>
T lcm(T x, T y) {
  T z = __gcd(x, y);
  return x * y / z;
}
template <typename T>
bool primejudge(T n) {
  if (n < 2)
    return false;
  else if (n == 2)
    return true;
  else if (n % 2 == 0)
    return false;
  double sqrtn = sqrt(n);
  for (T i = 3; i < sqrtn + 1; i++) {
    if (n % i == 0) {
      return false;
    }
    i++;
  }
  return true;
}
template <typename T>
T modinv(T a, T m) {
  T b = m, u = 1, v = 0;
  while (b) {
    T t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0) u += m;
  return u;
}
const ll dx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[8] = {0, 1, 1, 1, 0, -1, -1, -1};
template <class T>
inline void chmax(T &a, T b) {
  if (a < b) a = b;
}
template <class T>
inline void chmin(T &a, T b) {
  if (a > b) a = b;
}
static const double pi = 3.141592653589793;
int main() {
  cout << fixed << setprecision(30);
  double r;
  cin >> r;
  r *= pi;
  cout << r * 2 << endl;
  return 0;
}
