числа с тремя нулями

#include <iostream>

using namespace std;

unsigned long long  arr[32][4];

int main()
{
    int n;
    cin >> n;

    arr[1][0] = 1;
    for (int i = 2; i <= n; ++i)
    {
        for (int j = 0; j < 3; ++j)
        {
            arr[i][0] += arr[i-1][j];
        }
        arr[i][1] = arr[i-1][0];
        arr[i][2] = arr[i-1][1];
        arr[i][3] = arr[i-1][2] + 2*arr[i-1][3];
    }

    cout << arr[n][3];
    return 0;
}