#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>

using namespace std;

const int N = 300010;

typedef long long LL;

LL s[N];
deque<int> q;
int n, m;

int main()
{
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= n; i ++ )
    {
        cin >> s[i];
        s[i] += s[i - 1];
    }
    q.push_back(0);
    LL res = s[1];
    for (int i = 1; i <= n; i ++ )
    {
        if (q.front() < i - m) // 如果当前元素的下标与队头元素下标的距离超过m
            q.pop_front();
        if (!q.empty()) res = max(res, s[i] - s[q.front()]);
        while (!q.empty() && s[i] <= s[q.back()]) // 当前元素小于队尾元素
            q.pop_back();
        q.push_back(i);
    }
    printf("%d\n", res);
    return 0;
}