#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using pii = pair<int, int>;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  double R;
  cin >> R;
  cout << fixed << setprecision(10) << R * 2 * 3.141592 << "\n";
  return 0;
}
