use proconio::input;
fn main() {
    input! {
        n: usize,
        m: usize,
        mut a: [i64; n],
    }
    let mut b = vec![0; n + 1];
    for i in 0..n {
        if a[i] < 0 {
            a[i] = 0;
        }
        b[i + 1] += b[i] + a[i];
    }
    let mut ans: i64 = 0;
    for i in 0..=n - m {
        ans = ans.max(b[i + m] - b[i]);
    }
    println!("{}", ans);
}