#include <iostream>
using namespace std;
typedef long long LL;
const int N = 100010;
LL s[N];
LL n, k;
LL res;
int main()
{
    cin >> n >> k;
    for(int i = 1; i <= n; ++ i)
    {
        cin >> s[i];
        s[i] += s[i - 1];
    }
    for(int i = k; i <= n; ++ i)
    {
        res = max(res, s[i] - s[i - k]);
    }
    cout << res << '\n';
    return 0;
}