use proconio::*;

fn main() {
    input! {
        n: usize,
        m: usize,
        a: [usize; n],
        b: [usize; n],
    }
    let mut mon: Vec<usize> = vec![0; n];
    let mut ans: usize = 0;

    for (i, (x, &y)) in a.iter().zip(b.iter()).enumerate() {
        mon[i] = x.saturating_sub(y);
    }
    
    for i in mon.windows(m) {
        let x: usize = i.iter().sum();
        ans = ans.max(x);
    }
    println!("{ans}")
}