/*
yukicoder No.53(Ver2)
作成者：ヒロソフ
*/
#include <iostream>
#include <cstdio>
using namespace std;
double Calc(int k);

double Cache[98];
bool bWroteCache[98];

int main(void) {
	int N;
	for (int i = 0; i < 98; i++) {
		Cache[i] = 0;
		bWroteCache[i] = false;
	}
	scanf("%d", &N);
	cout << Calc(N) << endl;
	return 0;
}
double Calc(int k) {
	if (k == 0) {
		return 4;
	} else if (k == 1) {
		return 3;
	} else {
		if (bWroteCache[k - 2]) return Cache[k - 2];
		Cache[k - 2] = (19.0 * Calc(k - 1) - 12.0 * Calc(k - 2)) / 4.0;
		bWroteCache[k - 2] = true;
		return  Cache[k - 2];
	}
}