#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")
#pragma GCC optimization("O3")
#pragma GCC optimization("unroll-loops")
using namespace std;
template <typename A, typename B>
string to_string(pair<A, B> p);
string to_string(const string& s) { return '"' + s + '"'; }
string to_string(const char* s) { return to_string((string)s); }
string to_string(char& c) {
  string s;
  s += c;
  return s;
}
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (long long i = 0; i < static_cast<long long>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto& x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + " , " + to_string(p.second) + ")";
}
const long long MAXN = 1e6 + 7;
const long long MOD = 1e9 + 7;
const unsigned long long BASE = 311;
const long double EPS = 1e-9;
const long long INF = ((1 << 30) - 1);
const long long INFLL = ((1ll << 62ll) - 1ll);
const long long oo = 2e18;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  cout.setf(ios::fixed);
  cout.precision(10);
  long double N;
  cin >> N;
  cout << 2.0 * (acos(-1)) * N;
  return 0;
}
