递推

#include<iostream>
using namespace std;
int main(){
	int n,a[55];
	cin>>n;
	a[1]=3,a[2]=7;
	for(int i=3;i<=n;++i){
		a[i]=2*a[i-1]+a[i-2];
	}
	cout<<a[n];
	return 0;
}