------
## 最终结论
```python
def get_pell(n):
    if n == 0:
        return 0
    elif n == 1:
        return 1
    else:
        return 2 * get_pell(n - 1) + get_pell(n - 2)

assert get_pell(4) == 12
assert get_pell(7) == 169
assert get_pell(8) == 408
```