#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define endl "\n"
#define pll pair<ll,ll>
#define fs first
#define sc second

const ll MOD = 1e9+7;

void solve()
{
    ll n, k;
    cin >> n >> k;
    vector<ll> a(n);
    
    for(ll i = 0; i < n; i++)
    {
        cin >> a[i];
        a[i] += i;
    }
    
    if(k == 1)
    {
        cout << 0 << endl;
        return;
    }

    ll ans = LLONG_MAX;
    multiset<ll> elements;
    ll vsum = 0;

    for(ll i = 0; i < k; i++)
    {
        vsum += a[i];
        elements.insert(a[i]);
    }

    for(ll i = k-1; i < n; i++)
    {
        if(i >= k)
        {
            vsum -= a[i-k];
            vsum += a[i];
            elements.erase(elements.find(a[i-k]));
            elements.insert(a[i]);
        }
        auto it = elements.end();
        it--;
        ans = min(ans, k * *it - vsum);
    }
    cout << ans << endl;
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);
    ll t = 1;
    //cin >> t;
    while(t--)
    {
        solve();
    }
    return 0;
}