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.
Then, a value from arr was removed that was not the first or
last value in the array.
Return the removed value.
Example 1:
Input: arr = [5,7,11,13] Output: 9 Explanation: The previous array was [5,7,9,11,13].
Example 2:
Input: arr = [15,13,12] Output: 14 Explanation: The previous array was [15,14,13,12].
Constraints:
3 <= arr.length <= 10000 <= arr[i] <= 10^5