#include<bits/stdc++.h>
using namespace std;
int main() {
	//given an n length array and k, you eed to find the max subarray sum of length k.

	int n, k; cin >> n >> k;
	vector<int> v(n);
	for (int i = 0; i < n; i++) cin >> v[i];
	int ans = INT_MIN, tempsum = 0, s = 0;
	for (int i = 0; i < n; i++) {
		tempsum += v[i];
		if ((i - s + 1) == k) {
			ans = max(ans, tempsum);
			tempsum -= v[s];
			s++;
		}
	}
	cout << ans << endl;

	//follow-up questions
	/*
	 1.>find number of windows having maximum sum
	 2.>find the range of window having maxium sum.
	 3.>find the range of window having maximum sum, but it can be as nearer to the start.
	 4.>find the range of window having maximum sum, but it can be as nearer to the end.
	*/
	return 0;
}