#include <iostream>
#include <string>
#include <vector>
#include <iomanip>

#define rep(i,n) for (int i = 0; i < (n); ++i)

using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;

#define PI 3.14159263

int main() {
  double R;
  cin >> R;

  cout << std::fixed << std::setprecision(4) << 2 * R * PI << endl;

  return 0;
}
