#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

typedef pair<ll, ll> pii;

const ll INF = 4e18 + 7;

ll ind = 0;

struct comp{
    bool operator()(pii a, pii b){
        ll c = a.first + a.second - ind;
        ll d = b.first + b.second - ind;

        return c < d;
    }
};

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

    vector<ll> hegyek(n, 0);

    for(ll i = 0; i < n; i++){
        cin >> hegyek[i];
    }

    vector<ll> pref(n + 1, 0);
    for(ll i = 1; i <= n; i++){
        pref[i] = pref[i - 1] + hegyek[i - 1];
    }

    vector<ll> kezdo(n, 0);
    priority_queue<pii, vector<pii>, comp> sorrend;

    for(ll i = 0; i < k; i++){
        sorrend.push({hegyek[i], i});
    }

    for(ll i = 0; i + k - 1 < n; i++){
        ind = i;
        sorrend.push({hegyek[i + k - 1], i + k - 1});
        while(sorrend.top().second < i){
            sorrend.pop();
        }

        ll kesobb = sorrend.top().second - i;
        kezdo[i] = sorrend.top().first + kesobb;
    }

    ll mini = INF;
    for(ll i = 0; i + k - 1 < n; i++){
        ll koltseg = (kezdo[i] + kezdo[i] - k + 1) * k / 2 - (pref[i + k] - pref[i]);
        mini = min(mini, koltseg);
    }
    cout << mini << "\n";
}