#include <iostream>
using namespace std;

int arr[101010];
int main() {
    ios::sync_with_stdio(0);
	cin.tie(0);
    int n,k;
    cin >> n >> k;
    for (int i = 0;i < n;i++) {
        cin >> arr[i];
    }
    if (n == 1) {
        cout << arr[0];
        return 0;
    }
    int sum = 0;
    long long max = -10000000;
    for (int i = 0;i < k;i++) {
        sum += arr[i];
    }
    if (sum > max) {
        max = sum;
    }
    //cout << sum << " ";
    for (int i = 1;i < n;i++) {
        if (n-1 <= i+k-2) {
            break;
        }
        //cout << sum << " ";
        sum = sum - arr[i-1] + arr[i+k-1];
        if (sum > max) {
            max = sum;
        }
    }
    cout << max;
    return 0;
}