#include <iostream>
#include <vector>
#include <deque>
#include <climits>
using namespace std;

using ll = unsigned long long;
#define speed cin.tie(0); ios::sync_with_stdio(0)

vector<ll> heightS;
int N, K;

int main() {
    speed;

    cin >> N >> K;

    heightS.resize(N);

    for (int i = 0; i < N; i++) {
        cin >> heightS[i];
        heightS[i] += i;
    }

    deque<pair<int, int>> q;
    vector<ll> v(N - K + 1);
    
    for (int right = 0; right < K; right++) {
        while (!q.empty() && q.back().first < heightS[right]) q.pop_back();
        q.push_back({heightS[right], right});
    }

    v[0] = q.front().first;
    int p = 0;

    int left = 1, right = K;
    while (left < N - K + 1) {
        while (!q.empty() && q.front().second < left) q.pop_front();

        while (!q.empty() && q.back().first < heightS[right]) q.pop_back();

        q.push_back({heightS[right], right});

        left++;
        right++;

        v[++p] = q.front().first;
    }
    
    ll result = ULLONG_MAX;

    for (int i = 0; i < N - K; i++) {
        ll curr = 0;
        for (int j = i; j < i + K; j++) {

            curr += v[i] - heightS[j];
        }
        result = min(result, curr);
    }

    cout << result;
}