#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  double l;
  const double PI = 3.14159265358979323846;
  cin >> n;
  l = 2 * n * PI;
  cout << fixed << setprecision(8);
  cout << l << endl;
  cout << endl;
}
