#include<stdio.h>
#include<iostream>
#include<cstring>
#include<cmath>
using namespace std;
typedef long long ll;
int main()
{
ll a[51];
a[1] = 3;
a[2] = 6;
a[3] = 6;
for(int i=4;i<51;++i){
a[i] = a[i-1] + 2*a[i-2];
}
int n;
while(cin >> n && n){
cout << a[n] << endl;