/*
单调队列模板题
然而验题人在做题时用了dp解法
*/
#define method_1
#ifdef method_1
/*

*/
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<set>
#include<map>
#include<queue>
#include<stack>
#include<vector>
#include<cstring>
#include<cstdlib>
using namespace std;
typedef long long ll;
const int maxn=300000+5;
const int INF=0x3f3f3f3f;
int n,m;
int a[maxn],q[maxn];
int ans=-INF;
int main() {
    ios::sync_with_stdio(false);
    cin>>n>>m;
    for(int i=1;i<=n;i++){
        cin>>a[i];
        a[i]=a[i-1]+a[i];
    }
    int head=0,tail=0;
    q[head]=0;
    for(int i=1;i<=n;i++){
        while(head<=tail&&q[tail]-q[head]>=m) head++;
        ans=max(ans,a[i]-a[q[head]]);
        while(head<=tail&&a[q[tail]]>=a[i]) tail--;
        q[++tail]=i;
    }
    cout<<ans;
    return 0;
}
#endif
#ifdef method_2
/*

*/

#endif
#ifdef method_3
/*

*/

#endif