// 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()
{
    long long n, k;
    cin >> n >> k;
    vector<pair<long long,long long>>x(n + 1);
    priority_queue<pair<long long, long long>>p,p2;
    long long s1,s2,s3,mini=LONG_MAX,kulombseg;
    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 (p.top().first-x[i].first<i-p.top().second)
        {
            p2.push({ x[i].first,i });
            
        }
        while (!p2.empty() && p2.top().second <= i - k)
        {
            p2.pop();
        }
        if(!p2.empty())
        {
            if(p.top().first==p2.top().first&&p.top().second==p2.top().second) p2.pop();
            if(p.top().second>p2.top().second) p2.pop();
        }

        if (i >= k)
        {
            x[i].second -= x[i - k].first;
            s1 = (p.top().second - (i - k + 1)) + p.top().first;
            if(!p2.empty()) s2 = p.top().first - ((p2.top().second-1) - p.top().second);
            else s2 = p.top().first - (i - p.top().second);
            if (!p2.empty())
            {
                kulombseg = p2.top().first - s2+1;
                s2 = p2.top().first - (i - p2.top().second);
                s1 += kulombseg;
                s3 = (((s1 + s2) * k) / 2)-x[i].second;
                //cout<<s3<<"\n";
            }
            else
            {
                 s3 = (((s1 + s2)*k)/2)-x[i].second;
                 //cout<<s3<<"\n";
            }
            mini = min(mini, s3);
        }
    }
    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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