#include <cmath>

#include <algorithm>
#include <iostream>
#include <vector>
#include <map>
using namespace std;

#define inf 1000000000000000LL

long long dp[10010], c[10010];

int main() {
    // freopen("test.in", "r", stdin);
    int n, k;
    cin >> n >> k;
    for (int i = 1; i <= n; ++i) {
        cin >> c[i];
    }

    long long best = inf;

    for (int i = 1; i <= min(n, k+1); ++i) {
        int last = 0;
        long long sum = 0;
        for (int j = i; j <= n; j += 2*k+1) {
            last = j;
            sum += c[j];
        }

        if (last + k >= n) {
            best = min(best, sum);
        }
    }


    cout << best;
}