#include <iostream>

const int N = 1e6+10;

using namespace std;

int n, m;
long long w[N], s[N];

int main() {
    cin >> n >> m;
    for(int i = 1; i <= n; i++) cin >> w[i];
    for(int i = 1; i <= n; i++) s[i] = s[i-1] + w[i];

    long long res = 0;
    for(int i = m; i <= n; i++) 
        res = max(res, s[i] - s[i - m - 1]);
    cout << res << endl;
    return 0;
}