#include <bits/stdc++.h>
using namespace std;

typedef long long ll;

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);

    ll n, k;
    cin >> n >> k;

    vector<ll> vec(n);

    for (ll i=0; i < n; i++) cin >> vec[i];

    multiset<ll> ms;
    ll curr = 0;
    ll ans = LLONG_MAX;

    for (int i = 0; i < n; i++) {
        ms.insert(vec[i] + i);
        curr += vec[i];

        if (i >= k) {
            ms.erase(ms.find(vec[i - k] + (i - k)));
            curr -= vec[i - k];
        }

        if (i >= k - 1) {
            ll L = i - k + 1;
            ll H = *ms.begin() - L;

            ll reqHeight = k * H - k * (k - 1) / 2;
            ll cost = reqHeight - curr;
            if (cost >= 0)
                ans = min(ans, cost);
        }
    }

    cout << ans << '\n';

    return 0;
}