#include"bits/stdc++.h"
//#include<bits/stdc++.h>
using namespace std;
#define print(x) cout<<x<<endl;
#define rep(i,a,b) for(int i=a;i<b;i++)
typedef long long ll;
const ll mod = 2000000;

#define pi 3.141592

int main() {
	int n;
	cin >> n;
	double a[102] = {};
	a[0] = 4.0, a[1] = 3.0;
	rep(i, 2, n+1) {
		a[i] = (19.0000000000*a[i - 1] - 12.0000000000 * a[i - 2]) / 4.0000000000;
	}
	printf("%.10f\n", a[n]);
	return 0;
}