#include <iostream>
#include <algorithm>
#include <sstream>
#include <cmath>

using namespace std;

int main(int argc, char *argv[])
{
	cout.precision(12);
	int N;
	cin >> N;
	double A[101] = {}, A4[101] = {};
	A[0] = 4, A4[0] = 16;
	A[1] = 3, A4[1] = 12;
	for (int i = 2; i <= N; ++i) {
		A4[i] = 19 * A[i - 1] - 3 * A4[i - 2];
		A[i] = A4[i] * 0.25;
	}
	cout << A[N] << endl;
	return 0;
}