# Complete an Arithmetic Sequence

You are given a list of integers `nums`

that used to be an arithmetic sequence. Given that a number was removed, and that the number was not the first or the last element, return the removed number.

**Constraints**

`2 ≤ n ≤ 100,000`

https://binarysearch.com/problems/Complete-an-Arithmetic-Sequence

## Examples

### Example 1

**Input**

- nums =
`[1, 3, 5, 9]`

**Output**

- answer =
`7`

**Explanation**

If we add `7`

in to this sequence, we get `[1, 3, 5, 7, 9]`

which is an arithmetic sequence.

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