#include<iostream>
#include<iomanip>
using namespace std;
double d[111];
main()
{
	d[0]=4;
	d[1]=3;
	int N;cin>>N;
	for(int i=2;i<=N;i++)d[i]=(19*d[i-1]-12*d[i-2])/4;
	cout<<fixed<<setprecision(16)<<d[N]<<endl;
}