cp2019_zad2

#include <iostream>

using namespace std;
int numbers[1000001];

int main() {
    ios_base::sync_with_stdio(false);

    int m;
    while (cin >> m) {
        int n;
        cin >> n;

        signed long long maxSum = 0;
        signed long long currentSum = 0;


        for (int i = 0; i < m; i++) {
            cin >> numbers[i];
            currentSum += numbers[i];
        }

        maxSum = currentSum;

        for (int i = m; i < n; i++) {
            cin >> numbers[i];
            currentSum += numbers[i];
            currentSum -= numbers[i-m];
            if (currentSum > maxSum) {
                maxSum = currentSum;
            }
        }

        cout << maxSum << endl;

    }
}