#include <bits/stdc++.h>
using namespace std;
using vi = vector<int>;
using vii = vector<vi>;
using vs = vector<string>;
using in6 = int64_t;
using ind = double;
using pii = pair<int, int>;
using psi = pair<string, int>;
using vpi = vector<pii>;
using vps = vector<psi>;
void yes() { cout << "Yes"; }
void no() { cout << "No"; }
int main() {
  ios::sync_with_stdio(false);
  int a;
  cin >> a;
  const double pi = acos(-1.0);
  cout << 2 * a * pi;
  cout << "\n";
}
