First qualifying step window

A fitness challenge unlocks a badge the first time a member logs at least target total steps across k consecutive days. Given the daily list steps and the window length k (1 <= k <= number of days), return the 0-based start index of the FIRST window of k days whose sum reaches target, or -1 if it never happens. Slide a running sum and return as soon as the condition is met. Example: steps = [2000, 3000, 5000, 6000], k = 2, target = 9000 gives 2.