#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
const int INF = 1e9 + 7;
const long long LINF = 1LL << 60;
const double PI = 3.14159265358979323846;
int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  double r;
  cin >> r;
  cout << fixed << setprecision(24) << r * 2 * PI << endl;
  return 0;
}
