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 <= 1000 0 <= arr[i] <= 10^5