#nullable enable

#region
var (_input, _iter) = (Array.Empty<string>(), 0);
T I<T>() where T : IParsable<T>
{
    while (_iter >= _input.Length) (_input, _iter) = (Console.ReadLine()!.Trim().Split(' '), 0);
    return T.Parse(_input[_iter++], null);
}
#endregion

static T[] Range<T>(int n, Func<T> F) => Enumerable.Range(0, n).Select(_ => F()).ToArray();

var n = I<int>();
var m = I<int>();
var az = Range(n, I<int>);
var bz = Range(n, I<int>);

var sz = new long[n + 1];
for (var i = 0; i < n; i++) sz[i + 1] = sz[i] + Math.Max(0, az[i] - bz[i]);
var ans = 0L;
for (var i = m; i <= n; i++) ans = Math.Max(ans, sz[i] - sz[i - m]);
Console.WriteLine(ans);