#include <bits/stdc++.h>

using namespace std;

int main ()
{
    int n, k;
    cin >> n >> k;
    vector<long long> vec(n);
    for (int i = 0; i < n; i++) {
        cin >> vec[i];
        vec[i] += i;
    }
    multiset<long long> maxset;
    long long sum = 0;
    for (int i = 0; i < k; i++) {
        sum += vec[i];
        maxset.insert(vec[i]);
    }
    //for (auto x : maxset) cout << x << " ";
    //cout << "\n";
    long long mincost = INT64_MAX;
    for (int i = k - 1; i < n; i++) {
        if (i >= k) {
            sum -= vec[i - k];
            sum += vec[i];
            maxset.erase(maxset.find(vec[i - k]));
            maxset.insert(vec[i]);
        }
        auto it = maxset.end();
        it--;
        long long localmax = *it;
        long long cost = k * localmax - sum;
        if (cost < mincost) {
            mincost = cost;
        }
        /*cout << localmax << " " << sum << " " << cost << "\nset: ";
        for (long long x : maxset) {
            cout << x << " ";
        }
        cout << "\n";*/
    }
    cout << mincost;
}