啊题解

#include<bits/stdc++.h>
using namespace std;
int main(){
int n;
cin >> n;
long long a[50] = {};
a[1] = 0, a[2] = 6, a[3] = 6;
for(int i = 4; i <= n; i++){
a[i] = a[i-1] + a[i-2] * 2;
}
cout << a[n];
return 0;
}