#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
int n,m,s[300005],q[300005];
int main()
{
int i,ans=-1e9,h=0,t=0;
scanf("%d%d",&n,&m);
for(i=1;i<=n;i++)scanf("%d",&s[i]),s[i]+=s[i-1];
for(i=1;i<=n;i++){
if(q[h]<i-m)h++;
ans=max(ans,s[i]-s[q[h]]);
while(h<=t&&s[q[t]]>=s[i])t--;
q[++t]=i;
}
printf("%d\n",ans);
return 0;
}