#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>
using namespace std;

int n,t,a[100005];

int main()
{
    while(~scanf("%d%d",&n,&t))
    {
        for(int i = 1;i <= n;i++)    scanf("%d",&a[i]);
        int sum = 0;
        if(n <= t)
        {
            for(int i = 1;i <= n;i++)    sum += a[i];
            printf("%d\n",sum);
            continue;
        }
        for(int i = 1;i <= t;i++)    sum += a[i];
        int ans = sum;
        for(int l = 1,r = t+1;r <= n;l++,r++)
        {
            sum += a[r];
            sum -= a[l];
            ans = max(ans,sum);
        }
        printf("%d\n",ans);
    }
    return 0;
}