#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vl = vector<ll>;
using vs = vector<string>;
const double pi = 3.14159265358979;
const ll mod = 1000000007;
int main() {
  double N;
  cin >> N;
  cout << fixed << setprecision(10) << N * 2 * pi << endl;
}
