//留下的都是连续
#include<bits/stdc++.h>
using namespace std;
#define int long long
const int n=1e6+11;
int a,b[n],c;
signed main()
{
cin>>a>>c;
for(int i=1;i<=a;i++)
{
cin>>b[i];
}
int k=a-c;
int sum=0;
for(int i=1;i<=k;i++)
{
sum+=b[i];
}
int an=sum;
int p=1;
for(int i=k+1;i<=a;i++)
{
sum-=b[p];
sum+=b[i];
p++;
an=max(an,sum);
}
cout<<an<<endl;
}