#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  double n;
  cin >> n;
  double pi = 2 * acos(0.0);
  double res = 2 * pi * n;
  cout << fixed << cout.precision(2) << res << endl;
  return 0;
}
