\#include<iostream>
using namespace std;
const int N=300010,INF=-1e10;
int s[N],q[N];
int n,m;
int main()
{
cin>>n>>m;
for(int i=1;i<=n;i++){
cin>>s[i];
s[i]+=s[i-1];
}
int hh=0,tt=0;
q[0]=0;int res=INF;
for(int i=1;i<=n;i++){
if(q[hh]<i-m) hh++;
res=max(res,s[i]-s[q[hh]]);
while(hh<=tt&&s[q[tt]]>s[i]) tt--;
q[++tt]=i;
}
cout<<res<<endl;
return 0;
}