Best training block

A step-tracking app highlights a user’s best training block. Given daily step counts steps (non-empty) and a block length k (1 <= k <= number of days), return the 0-based start index of the k-day stretch with the highest total steps. If several stretches tie, return the earliest. Maintain a running window sum instead of re-adding k values per position. Example: steps = [4000, 9000, 6000, 7000, 2000], k = 2 gives 1.