#include <bits/stdc++.h>
using namespace std;
long long int ar[500009], br[500009], cr[500009];
map<long long int, long long int> mp, mp1;
vector<long long int> v;
string s, d;
int main() {
  double q, r;
  cin >> q;
  r = (2.0 * q * acos(1.0));
  cout << fixed << setprecision(6) << r << endl;
}
