#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vl = vector<ll>;
#define pb push_back
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define all(v) (v).begin(), (v).end()
int n=7;

int main() {
    int n,m;
    cin >> n >> m;
    vl A(n);
    rep(i,n)cin >> A[i];
    rep(i,n){
        int b;
        cin >> b;
        A[i]=max(A[i]-b,(ll)0);
    }
    ll ans=0;
    ll sum=0;
    rep(i,m)sum+=A[i];
    ans=sum;
    rep(i,n-m){
        sum+=A[m+i];
        sum-=A[i];
        ans=max(ans,sum);
    }
    cout << ans << "\n";
}