#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    int n, k;
    cin >> n >> k;
    const long long int INF = 1000000000000000;
    vector<long long int> cost(n);
    for (size_t i = 0; i < n; ++i) {
        cin >> cost[i];
    }
    //incorrect borders!!
    long long int leap = 2 * k + 1;
    vector<long long int> total_price(leap, 0);
    for (int l = 0; l <= k; ++l) {
        int j = 0;
        while (l + j * leap < n) {
            total_price[l] += cost[l + j * leap];
            ++j;
        }
        if (l + (j - 1) * leap + k + 1 < n) {
            total_price[l] += INF;
        }        
    }
    long long int result = INF;
    for (size_t i = 0; i <= k; ++i) {
        result = min(result, total_price[i]);
    }
    cout << result << endl;
    return 0;
}