#include <bits/stdc++.h>
using namespace std;

int main() {
    int N, M;
    cin >> N >> M;
    vector<int> A(N);
    for (int i = 0; i < N; ++i) {
        cin >> A[i];
    }

    long long max_sum = LLONG_MIN;
    long long current_sum = 0;
    deque<int> dq;

    for (int i = 0; i < N; ++i) {
        current_sum += A[i];
        dq.push_back(A[i]);

        if (dq.size() > M) {
            current_sum -= dq.front();
            dq.pop_front();
        }

        if (current_sum > max_sum) {
            max_sum = current_sum;
        }
    }

    cout << max_sum << endl;
    return 0;
}