代码：

import java.io.*;
class Main{
    static int N = 300010;
    static BufferedReader read = new BufferedReader(new InputStreamReader(System.in));
    static int[] a = new int[N], h = new int[N], s = new int[N];
    public static void main(String[] args) throws Exception{
        String[] ss = read.readLine().split(" ");
        int n = Integer.valueOf(ss[0]), m = Integer.valueOf(ss[1]);
        ss = read.readLine().split(" ");
        for(int i = 1; i <= n; i++) {
            a[i] = Integer.valueOf(ss[i - 1]);
            s[i] = s[i - 1] + a[i];   //求前缀和
        }
        int hh = 0, tt = -1, ans = Integer.MIN_VALUE;
        for(int i = 1; i <= n; i++){
            if( tt >= hh && i - 1 - h[hh] + 1 > m) hh++;
            while(tt >= hh && s[h[tt]] >= s[i - 1]) tt--;
            h[++tt] = i - 1;   //每次将i - 1的下标入队
            ans = Math.max(s[i] - s[h[hh]], ans);
        }
        System.out.println(ans);
    }
}