use proconio::input;

fn main() {
    input! {
        n: usize, m: usize,
        a: [i64; n],
        b: [i64; n],
    }

    let mut ans = 0;
    let mut sum = 0;
    for i in 0..n {
        sum += (a[i] - b[i]).max(0);

        if i >= m - 1 {
            ans = ans.max(sum);
            sum -= (a[i + 1 - m] - b[i + 1 - m]).max(0);
        }
    }
    println!("{ans}");
}