#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
template <typename T>
using vec2d = vector<vector<T>>;
template <typename T>
using vec3d = vector<vector<vector<T>>>;
template <typename T>
using vec4d = vector<vector<vector<vector<T>>>>;
template <typename T>
using vec5d = vector<vector<vector<vector<vector<T>>>>>;
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &v : vec) is >> v;
  return is;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "[";
  for (auto it = vec.begin(); it != vec.end(); it++) {
    os << *it;
    auto it2 = it;
    it2++;
    if (it2 != vec.end()) {
      cout << ",";
    }
  }
  os << "]";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const deque<T> &vec) {
  os << "deq[";
  for (auto v : vec) os << v << ",";
  os << "]";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const set<T> &vec) {
  os << "{";
  for (auto v : vec) os << v << ",";
  os << "}";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &vec) {
  os << "{";
  for (auto v : vec) os << v << ",";
  os << "}";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const multiset<T> &vec) {
  os << "{";
  for (auto v : vec) os << v << ",";
  os << "}";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_multiset<T> &vec) {
  os << "{";
  for (auto v : vec) os << v << ",";
  os << "}";
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &pa) {
  os << "(" << pa.first << "," << pa.second << ")";
  return os;
}
template <typename TK, typename TV>
ostream &operator<<(ostream &os, const map<TK, TV> &mp) {
  os << "{";
  for (auto v : mp) os << v.first << "=>" << v.second << ",";
  os << "}";
  return os;
}
template <typename TK, typename TV>
ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp) {
  os << "{";
  for (auto v : mp) os << v.first << "=>" << v.second << ",";
  os << "}";
  return os;
}
template <typename T>
void ndarray(vector<T> &vec, int len) {
  vec.resize(len);
}
template <typename T, typename... Args>
void ndarray(vector<T> &vec, int len, Args... args) {
  vec.resize(len);
  for (auto &v : vec) ndarray(v, args...);
}
template <typename T>
bool chmax(T &m, const T q) {
  if (m < q) {
    m = q;
    return true;
  } else
    return false;
}
template <typename T>
bool chmin(T &m, const T q) {
  if (q < m) {
    m = q;
    return true;
  } else
    return false;
}
template <typename T1, typename T2>
pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) {
  return make_pair(l.first + r.first, l.second + r.second);
}
template <typename T1, typename T2>
pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) {
  return make_pair(l.first - r.first, l.second - r.second);
}
template <typename T>
vec2d<T> make2d(lint L, lint M, T value) {
  vec2d<T> ans(M, vector<T>(L, value));
  return ans;
}
template <typename T>
vec3d<T> make3d(lint L, lint M, lint N, T value) {
  vec3d<T> ans(N, vec2d<T>(M, vector<T>(L, value)));
  return ans;
}
template <typename T>
vec4d<T> make4d(lint L, lint M, lint N, lint O, T value) {
  vec4d<T> ans(O, vec3d<T>(N, vec2d<T>(M, vector<T>(L, value))));
  return ans;
}
template <typename T>
vec5d<T> make5d(lint L, lint M, lint N, lint O, lint P, T value) {
  vec5d<T> ans(O, vec4d<T>(O, vec3d<T>(N, vec2d<T>(M, vector<T>(L, value)))));
  return ans;
}
template <typename T>
void setAll1d(vector<T> &vec, T value) {
  for (auto &v : vec) {
    v = value;
  }
}
template <typename T>
void setAll2d(vec2d<T> &vec, T value) {
  for (auto &v1 : vec) {
    for (auto &v2 : v1) {
      v2 = value;
    }
  }
}
template <typename T>
void setAll3d(vec3d<T> &vec, T value) {
  for (auto &v3 : vec) {
    for (auto &v2 : v3) {
      for (auto &v1 : v2) {
        v1 = value;
      }
    }
  }
}
template <typename T>
void setAll4d(vec4d<T> &vec, T value) {
  for (auto &v4 : vec) {
    for (auto &v3 : v4) {
      for (auto &v2 : v3) {
        for (auto &v1 : v2) {
          v1 = value;
        }
      }
    }
  }
}
template <typename T>
void setAll5d(vec5d<T> &vec, T value) {
  for (auto &v5 : vec) {
    for (auto &v4 : v5) {
      for (auto &v3 : v4) {
        for (auto &v2 : v3) {
          for (auto &v1 : v2) {
            v1 = value;
          }
        }
      }
    }
  }
}
template <typename T>
void vout2d(vec2d<T> vec) {
  cout << "[" << endl;
  for (auto(v) : (vec)) {
    cout << "  " << v << endl;
  }
  cout << "]" << endl;
}
template <typename T>
void vout3d(vec2d<T> vec) {
  cout << "[" << endl;
  for (auto(v) : (vec)) {
    cout << "  [" << endl;
    for (auto(x) : (v)) {
      cout << "    " << x << endl;
    }
    cout << "  ]" << endl;
  }
  cout << "]" << endl;
}
lint gcp(lint a, lint b) {
  if (a > b) return (gcp(b, a));
  if (a == 0) return (0);
  lint r = 1;
  while (1) {
    r = b % a;
    if (r == 0) break;
    b = a;
    a = r;
  }
  return (a);
}
lint factorial(lint n, lint mod) {
  if (n <= 1) return 1;
  if (mod == 0) return (n * factorial(n - 1, 0));
  return (n * factorial(n - 1, mod) % mod);
}
lint power(lint a, lint b, lint p) {
  if (b == 0) {
    return 1;
  } else if (b % 2 == 0) {
    lint d = power(a, b / 2, p);
    if (p == 0)
      return (d * d);
    else
      return (d * d) % p;
  } else {
    if (p == 0)
      return (a * power(a, b - 1, 0));
    else
      return (a * power(a, b - 1, p)) % p;
  }
}
lint nget(lint num, lint n) {
  return (num % power(10, n, 0) / power(10, n - 1, 0));
}
lint modinv(lint n, lint mod) { return (power(n, mod - 2, mod)); }
lint factorialinv(lint n, lint mod) {
  if (n == 1)
    return 1;
  else
    return modinv(n, mod) * factorialinv(n - 1, mod);
}
lint comb(lint n, lint m, lint mod) {
  return factorial(n, mod) * factorialinv(m, mod) % mod *
         factorialinv(n - m, mod) % mod;
}
int main() {
  const double PI = 3.14159265358979323846;
  double r;
  cin >> r;
  cout << 2 * r * PI << endl;
  return 0;
}
