package no3684_chokudai_niku_png;
import java.util.*;
public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int m = sc.nextInt();
		long[] psum = new long[n + 1];
		long[] a = new long[n + 1];
		long[] b = new long[n + 1];
		for(int i = 1;i <= n;i++) {
			a[i] = sc.nextLong();
		}for(int i = 1;i <= n;i++) {
			b[i] = sc.nextLong();
			psum[i] = psum[i - 1];
			psum[i] += Math.max(0, a[i] - b[i]);
			//System.out.print(psum[i] + " ");
		}
		long ans = Long.MIN_VALUE;;
		for(int i = m;i <= n;i++) {
			ans = Math.max(ans, psum[i] - psum[i - m]);
		}System.out.println(ans);
	}

}