#include <bits/stdc++.h>

using namespace std;

int main() {
  int n;
  cin >> n;
  vector<double> a{4, 3};
  a.resize(n + 1);
  for (int i = 2; i <= n; ++i) a[i] = (19 * a[i - 1] - 12 * a[i - 2]) / 4;
  cout << fixed << setprecision(15) << a[n] << endl;
}