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# 1228. Missing Number In Arithmetic Progression

<https://leetcode.com/problems/missing-number-in-arithmetic-progression>

## Description

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*.

**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] <= 105`
* The given array is **guaranteed** to be a valid array.

## ac

```java
```