#include <bits/stdc++.h>

using namespace std;

const long long INF = (long long) 1e18;
const int N = 1e4 + 1;

int main() {
    int n, k;
    cin >> n >> k;
    vector<int> a(n + 1);
    for (int i = 1; i <= n; ++i) {
        cin >> a[i];
    }


    long long ans = INF;
    for (int x = 1; x <= k + 1; ++x) {
        for (int y = n; y >= n - k; --y) {
            int L = x + k + 1;
            int R = y - k - 1;
            if (L >= R) {
                ans = min(ans, 1LL * a[x] + a[y] * (x != y));
                continue;
            }
            if ((R - L + 1) % (k + k + 1) == 0) {
                //cerr << L << ' ' << R << endl;

                long long cur = 0;
                for (int i = L + k; i <= R; i += k + k + 1)
                    cur += a[i];
                ans = min(ans, cur + a[x] + a[y] * (x != y));
            }
        }
    }
    cout << ans << endl;
    return 0;
}