#include <bits/stdc++.h>
using namespace std;
int main(void){
    int n,m;
    cin>>n>>m;
    vector<long long>a(n),ar(n+1,0);
    for(int i=0;i<n;i++)cin>>a[i];
    for(int i=0;i<n;i++){
        int b;
        cin>>b;
        a[i]-=b;
        a[i]=max(a[i],0LL);
        ar[i+1]=a[i]+ar[i];
    }
    long long ans=0;
    for(int i=0;i<n-m+1;i++){
        ans=max(ans,ar[i+m]-ar[i]);
    }
    cout<<ans<<endl;
    return 0;
}