#include <bits/stdc++.h>
using namespace std;
void p_() { cout << "\n"; }
template <class Head, class... Body>
void p_(Head head, Body... body) {
  cout << head << ",";
  p_(body...);
}
int main(int argc, char const *argv[]) {
  int R;
  cin >> R;
  cout << R * 2 * 3.14159 << endl;
  return 0;
}
