Please solve the programming task below using a self-contained code snippet in a markdown code block.

Given an array of distinct integers arr, where arr is sorted in ascending order, return the smallest index i that satisfies arr[i] == i. If there is no such index, return -1.
 
Please complete the following python code precisely:
```python
class Solution:
    def fixedPoint(self, arr: List[int]) -> int:
```