#include<bits/stdc++.h>
#define ll long long
using namespace std;
ll n, m, ans = -2147483648, sum, len;
vector<ll> a;
int main() {
	scanf("%lld%lld", &n, &m), a.resize(n + 1);
	for (int i = 1; i <= n; ++i) {
		scanf("%lld", &a[i]), sum += a[i];
		if (len == m) sum -= a[i - len];
		else len++;
		ans = max(ans, sum);
		if (sum < 0) sum = 0, len = 0;
	}
	cout << ans;
	return 0;
}