#include <iostream>
#include <iomanip>
using namespace std;

int main(){
	int n;
	cin >> n;
	long double a[101];
	a[0] = 4;
	a[1] = 3;
	for(int i = 2; i <= 100; i++) a[i] = (19 * a[i - 1] - 12 * a[i - 2]) / 4;
	cout << fixed << setprecision(10) << a[n] << endl;
}