#include <bits/stdc++.h>

using namespace std;

int main()
{
    //ifstream cin("in.txt");
    long long n;
    cin >> n;
    long long k;
    cin >> k;
    vector<long long> heights(n + 2);
    vector<long long> ps(n + 1);
    for(int i = 1; i <= n; i++){
        cin >> heights[i];
        ps[i] = ps[i - 1] + heights[i];
    }

    multiset<long long> s;
    for(int i = 1; i <= k; i++){
        s.insert(heights[i] + i);
    }

    long long ans = INT_MAX;
    for(long long i = k; i <= n; i++){
        long long actualsum = ps[i] - ps[i - k];
        long long neededsum = (*s.rbegin()- i) * k + (k - 1)*k/2;
        //cout << i << " " << heights[i] << " " << *s.rbegin() - i << " " << neededsum - actualsum << '\n';
        ans = min(ans, neededsum - actualsum);
        s.erase(s.lower_bound(heights[i - k + 1] + i - k + 1));
        s.insert(heights[i + 1] + i + 1);
    }
    cout << ans;



    return 0;
}