use proconio::input;

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

    let mut s = vec![0_i64; n+1];
    for i in 0..n{
        s[i+1] = s[i]+(a[i]-b[i]).max(0_i64);
    }

    let mut ans = i64::MIN;
    for i in m..=n{
        ans = ans.max(s[i]-s[i-m]);
    }

    println!("{}", ans);
}