Maximum k-Sum Window

**Description:** Given n numbers, find the maximum sum of any k **consecutive** numbers (a fixed window, not any subset). **Input:** Line 1: n k (1 ≤ k ≤ n ≤ 10^5). Line 2: n numbers (−10^9..10^9). **Output:** One integer. **Example:** 5 2 / 3 -1 4 2 0 → 6 (window 4+2). *Note:* an O(n·k) double loop passes only small n here — but the sliding sum (add the new element, drop the old) is O(n). Try both; one times out at n = 10^5.