#include <bits/stdc++.h>
using namespace std;
using Int = long long;
int PREP = (cin.tie(nullptr), ios::sync_with_stdio(false),
            cout << fixed << setprecision(9), 0);
int main() {
  double R;
  cin >> R;
  cout << 2 * acos(-1) * R << endl;
  return 0;
}
