#include <bits/stdc++.h>
using namespace std;
bool func_to_create() {
  unsigned int R;
  cin >> R;
  float ans = R * 2 * M_PI;
  cout << ans << endl;
}
int main() {
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(NULL);
  func_to_create();
  return 0;
}
