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

typedef long long ll;

int n, k;
ll c[10010], dp[10010];

int D = 1000;

int main() {
    cin >> n >> k;
    
    for (int i = 0; i < n; i++)
        cin >> c[i];
    
    for (int i = n-1; i >= 0; i--)
        dp[i] = c[i] + (i + 2 * k + 1 < n ? dp[i+2*k+1] : 0);
    
    ll ans = 1LL<<60;
    for (int i = 0; i <= min(k, n); i++){
        int rem = n - min(n, i + k + 1);
        if (rem == 0 || rem % (2*k-1) >= k+1)
            ans = min(ans, dp[i]);
    }
    
    cout << ans << endl;
}