#include <iostream>
#include <vector>
using namespace std;
typedef long long LL;

int main(int argc, char* argv[])
{
	vector<LL> f2(51);
	vector<LL> f1(51);
	f2[3]=1;
	f2[4]=2;
	f1[3]=1;
	f1[4]=1;
	int n;
	for (n=5;n<=50;n++){
		f2[n]=f2[n-1]+f2[n-2];
		f1[n]=f1[n-1]+f1[n-2];
	}

	int N;
	cin>>N;
	
	cout<<2*f2[N]+f1[N]<<endl;

	return 0;
}