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

In some array arr, the values were in arithmetic progression: the values arr[i + 1] - arr[i] are all equal for every 0 <= i < arr.length - 1.
A value from arr was removed that was not the first or last value in the array.
Given arr, return the removed value.
 
Please complete the following python code precisely:
```python
class Solution:
    def missingNumber(self, arr: List[int]) -> int:
```