#include <bits/stdc++.h>
using namespace std;
#define ll long long int
const int M = 1e9 + 7;
 
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int n, k;
    cin >> n >> k;
    vector<int> v(n);
    int tot = 0;
    for (int i = 0; i < n; i++)
    {
        cin >> v[i];
        tot += v[i];
    }
    int l = 0, r = 0;
    int mi = INT_MAX, sum = 0;
    while (r < n)
    {
        sum += v[r];
        if (r - l + 1 == k)
        {
            mi = min(mi, sum);
            if (sum > 0)
                sum -= v[l];
            l++;
        }
        r++;
    }
 
    cout << tot - mi + (mi * (mi + 1) / 2) << endl;
    return 0;
}

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OSA0CjEgMSAwIDAgMSAwIDEgMCAx

9 4
1 1 0 0 1 0 1 0 1