// sipalya'.cpp : This file contains the 'main' function. Program execution begins and ends there.
//

#include <iostream>
#include <deque>
#include <algorithm>
#include <vector>
#include <queue>
#include <climits>

using namespace std;

int main()
{
    int n, k;
    cin >> n >> k;
    vector<pair<int,int>>x(n + 1);
    priority_queue<pair<int, int>>p;
    int s1,mini=INT_MAX;
    for (int i = 1; i <= n; ++i)
    {
        cin >> x[i].first;
        x[i].second = x[i - 1].second + x[i].first;
        p.push({ x[i].first,i });
        while (!p.empty() && p.top().second <= i - k)
        {
            p.pop();
        }
        if (i >= k)
        {
            x[i].second -= x[i - k].second;
            s1 = (((((p.top().second - (i - k + 1)) + p.top().first) + (p.top().first - (i - p.top().second))) * k) / 2)-x[i].second;
            mini = min(mini, s1);
        }
    }
    cout << mini;
    return 0;
}
/*
5 3
5 5 6 3 1
*/
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// Debug program: F5 or Debug > Start Debugging menu

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