Given a list of integers values, your job is to return the sum of the values; however, if the same integer value appears multiple times in the list, you can only count it once in your sum.
For example:
```python
[ 1, 2, 3] ==> 6
[ 1, 3, 8, 1, 8] ==> 12
[ -1, -1, 5, 2, -7] ==> -1
[] ==> None
```
Good Luck!
def unique_sum(lst):
    return sum(set(lst)) if len(lst) > 0 else None
def unique_sum(lst):
    return sum(set(lst)) if len(lst) > 0 else None