#include <algorithm>
#include <iostream>
#include <string>
using namespace std;
#define PI 3.14159265359
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
int main() {
  int R;
  cin >> R;
  double ans = 2 * PI * R;
  cout << ans << endl;

  return 0;
}
