题解

#include<bits/stdc++.h>
using namespace std;

int main()
{
	long n;
	cin >> n;
	long a[n+1];
    a[1] = 0;
    a[2] = 6;
    for(long long i=3; i<=n; i++)
    {
        a[i] = a[i-1] + a[i-2] * 2;
    }
    
    cout << a[n];
    return 0;
}