#include <bits/stdc++.h>
using namespace std;
using ll = long long;
ll solve() {
  ll R;
  cin >> R;
  cout << R * M_PI * 2 << "\n";
  return 0;
}
int main() {
  cout.setf(ios::fixed);
  cout.precision(16);
  solve();
  return 0;
}
