#include <bits/stdc++.h>
using namespace std;
const bool testcases = 0;
long long seed;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
inline long long rnd(long long l = 0, long long r = INT_MAX) {
  return uniform_int_distribution<long long>(l, r)(rng);
}
template <typename T>
bool isPrime(T n) {
  for (T i = 2; i * i <= n; i++)
    if (n % i == 0) return 0;
  return 1;
}
template <typename T>
inline bool in_range(T a, T l, T r) {
  return (a >= l && a <= r);
}
template <typename T, typename U>
inline istream &operator>>(istream &in, pair<T, U> &a) {
  in >> a.first >> a.second;
  return in;
}
template <typename T>
inline istream &operator>>(istream &in, vector<T> &a) {
  for (auto &x : a) in >> x;
  return in;
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  long long r;
  long long TC = 1;
  if (testcases) cin >> TC;
  for (long long tc = 1; tc <= TC; tc++) {
    cout << 6.28318530717958623200 * r << '\n';
    108;
  }
  return 0;
}
