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

typedef long long ll;

const ll INF = (ll)1e18;
const int N = (int)1e4 + 10;
int n, k;
ll a[N];

int main() {
    
    scanf("%d%d", &n, &k);
    for (int i = 0; i < n; i++)
        scanf("%lld", &a[i]);
    ll ans = INF;
    for (int i = 0; i <= k && i < n; i++)
        {
        int lst = i;
        ll sum = 0;
        for (int j = i; j < n; j += 2 * k + 1)
            {
            sum += a[j];
            lst = j;
        }
        if (lst + k < n - 1) continue;
        ans = min(ans, sum);
    }
    printf("%lld\n", ans);
    
    return 0;
}