#include <bits/stdc++.h>
using namespace std;
long long qs[1000100];
int maxx[1000100];
int main(){

    int n,k;
    scanf(" %d %d",&n,&k);

    for(int i = 1;i<=n;i++){
        scanf(" %lld",&qs[i]);
        qs[i] += qs[i-1];

    }
    /*
    cout << endl;
    for (int i = 1;i<=n;i++) cout << qs[i] << " ";
    cout << endl;
    */
    for(int i = n;i>=1;i--){
        maxx[i] = i;
        if(i!=n+1 && qs[maxx[i+1]] >= qs[i]) maxx[i] = maxx[i+1];
    }
    /*
    for (int i = 1;i<=n;i++) cout << maxx[i] << " ";
    cout << endl;
    */
    long long ans = -1e18;
    long long minn = 1e18;
    for(int i = k;i<=n;i++){
        minn = min(minn,qs[i-k]);
        ans = max(ans,qs[maxx[i]] - minn);
        //cout << minn << " "  << ans << endl;
    }
    printf("%lld",ans);




}