#include <bits/stdc++.h>
using namespace std;
constexpr int inf = 1000000000;
constexpr long long INF = 1000000000000000000;
constexpr long double EPS = 1e-12;
constexpr long double PI = 3.141592653589793238;
template <class T, class U>
inline bool chmax(T &a, const U &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T, class U>
inline bool chmin(T &a, const U &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(20);
  double r;
  cin >> r;
  cout << r * 6.283185307 << '\n';
}
