#include <bits/stdc++.h>

using namespace std;

typedef long long int ll;

const int NMax = 5e5 + 5;
const int LIM = 1e9;

int v[10005];

int main(){

    ios::sync_with_stdio(false);
    cin.tie(NULL);

    int n, m;
    cin >> n >> m;

    for(int i = 1; i <= n; i++) cin >> v[i];

    ll total, ans;
    ans = INFINITY;
    for(int i = 1; i <= m + 1; i++){
        total = 0;

        int j;
        for(j = i; j <= n; j += 2 * m + 1) total += v[i];

        if(j - 2 * m - 1 < n - m) total = INFINITY;

        ans = min(ans, total);

    }

    cout << ans;

    return 0;

}