#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1);
int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  int R;
  cin >> R;
  cout << fixed << setprecision(10) << 2.0 * pi * R << "\n";
  return 0;
}
