#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-10;
using ll = long long;
using Pi = pair<int, int>;
using Pl = pair<ll, ll>;
int main() {
  cout << fixed << setprecision(5);
  int r;
  cin >> r;
  double ans = 2 * r * acos(-1.0);
  ;
  cout << ans << endl;
}
