#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using Graph = vector<vector<int>>;
const double PI = 3.1415926535;
int main() {
  double r;
  cin >> r;
  double ans = 2.0 * PI * r;
  cout << fixed << setprecision(5) << ans << endl;
}
