#include <bits/stdc++.h>
using namespace std;
int main() {
  ;
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  ;
  double r;
  cin >> r;
  r = 2 * r;
  r = r * (3.1415926);
  cout << fixed << setprecision(5) << r << endl;
  return 0;
}
