public static int minimumTotalCost(int[] departures, int[] returns, int minGap) {
        int res = Integer.MAX_VALUE;
        int minDeparture = Integer.MAX_VALUE; // Tracks minimum departure in valid window

        for (int R = 0; R < departures.length; R++) {
            // Only update minDeparture when we reach a valid window
            if (R >= minGap) {
                minDeparture = Math.min(minDeparture, departures[R - minGap]);
                res = Math.min(res, returns[R] + minDeparture);
            }
        }

        return res == Integer.MAX_VALUE ? -1 : res;
    }