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

#include <iostream>
#include <queue>

using namespace std;

int n, k, i, m=1000000000, s, ns, u, e;
vector<int>x,pf;
vector<pair<int, int>>v;
priority_queue < pair<int, int>>pq;

int main()
{
    cin >> n >> k;

    x.resize(n + 1);
    pf.resize(n + 1);

    for (i = 1; i <= n; ++i)
    {
        cin >> x[i];
        pq.push({ x[i],i });
        pf[i] = x[i] + pf[i - 1];
    }

    for (i = k; i <= n; ++i)
    {
        s = pf[i] - pf[i - k];
        v.clear();
        while (pq.top().second < (i - k + 1) && pq.top().second > i)
        {
            v.push_back(pq.top());
            pq.pop();
        }
        
        e = pq.top().first + (pq.top().second - (i - k + 1));
        u = e - k + 1;
        ns = ((e + u) * k) / 2;
        for (auto& e : v) pq.push(e);

        if ((ns - s) < m) m = ns - s;
    }

    cout << m;
}

/*
5 3
5 5 6 3 1
*/

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