#include <bits/stdc++.h>
using ll = long long;
using namespace std;
const auto mod = 1e9 + 7;
const long double pi = (acos(-1));
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(20);
  long double r;
  cin >> r;
  cout << 2 * r * pi << endl;
}
