#include <stdio.h>
#include <algorithm>
#define INF 100000000000000000LL
#define MAX 10005

typedef long long ll;

ll c[MAX], pre[MAX];

using namespace std;

int main() {
    ll n, K, i, ans, rem;
    scanf("%lld%lld", &n, &K);
    for (i = 0; i < n; i++) {
        scanf("%lld", &c[i]);
    }
    for (i = n - 1 ; i >= 0; i--) {
        if (i + 2 * K + 1 < n) {
            pre[i] = pre[i + 2 * K + 1] + c[i];
        } else {
            pre[i] = c[i];
        }
    }
    ans = INF;
    for (i = 0; i <= K && i < n; i++) {
        rem = n - i - 1 - K;
        if (rem < 0) {
            ans = min(ans, c[i]);
        } else if ((rem % (2 * K + 1)) == 0 || (rem % (2 * K + 1)) >= K + 1) {
            ans = min(ans, pre[i]);
        }
    }
    printf("%lld\n", ans);
    return 0;
}