An integer array arr was originally an arithmetic progression of numbers, but exactly one integer was removed. The first and last elements of the original progression were never removed.

Return the missing integer.

Constraints:

• $3 \leq$ arr.length $\leq 1000$

• $0 \leq$ arr[i] $\leq 10^5$

• The given array is guaranteed to be a valid array.

Now, let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Implement your solution in the following coding playground.