Compute a key value from a list of numbers using a repeated rule over adjacent values.
You are given an array of integers. Starting from the first element, compare neighboring values and derive a new value according to the problem's key rule. Continue processing the array until a single key value can be determined.
Your task is to return that final key. The exact transformation is intentionally framed as a practice exercise for careful array processing and arithmetic handling.
nums.Example 1
Input
nums = [5, 2, 9, 1]
Output
1
Explanation
Illustrative example: apply the key rule across the numbers from left to right until a single value remains.
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