#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
using pii = pair<int, int>;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr int mod1 = 1e9 + 7;
constexpr int mod2 = 998244353;
template <class A, class B>
inline bool chmax(A &a, const B &b) {
  return b > a && (a = b, true);
}
template <class A, class B>
inline bool chmin(A &a, const B &b) {
  return b < a && (a = b, true);
}
template <class T>
using min_heap = priority_queue<T, vector<T>, greater<T> >;
template <class T>
using max_heap = priority_queue<T>;
template <class A, class B>
using umap = unordered_map<A, B>;
inline int square(int a) { return a * a; }
inline int updiv(int a, int b) { return (a + b - 1) / b; }
template <class A, class B>
inline int Max(const A &a, const B &b) {
  return (a < b ? b : a);
}
template <class A, class B>
inline int Min(const A &a, const B &b) {
  return (a > b ? b : a);
}
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
constexpr int dx[] = {1, 0, -1, 0, 1, 1, -1, -1};
constexpr int dy[] = {0, -1, 0, 1, 1, -1, -1, 1};
signed main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  int r;
  cin >> r;
  cout << r * 2 * M_PI << '\n';
  return 0;
}
