https://www.codecademy.com/courses/learn-python-3/articles/python-code-challenges-lists

hey guys, I’m doing this optional challenge for python 3, I do not quite understand the second challenge: append sum,

here is my solution,

```
def append_sum(lst):
lst.append(lst[-2:])
lst.append(lst[-2:])
lst.append(lst[-2:])
return lst
```

but I get result

`[1, 1, 2, [1, 2], [2, [1, 2]], [[1, 2], [2, [1, 2]]]]`

the correct answer use:

`lst.append(lst[-1]+lst[-2])`

I don’t understand why my solution doesn’t work, and what it the difference?

thank you in advance.