K Radius Subarray Averages

You're smoothing a noisy sensor log: for each reading, average it with the k readings on either side, rounded down, or answer -1 when one side runs out of neighbors. Interviewers love it because each window overlaps the next one almost entirely. The classic trap is overflow: a few readings near two billion already outgrow a 32-bit integer. You get 1 to 100,000 readings, each from 0 to 2,000,000,000, and k can be anything from 0 to 100,000.

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