#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
void Main() {
  int R;
  cin >> R;
  cout << 2.0 * acos(-1) * R << endl;
}
int main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(20);
  Main();
  return 0;
}
