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


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */
    ll n, k;
    cin >> n >> k;
    vector<ll> cs;
    ll c;
    for(ll i = 0; i < n; i++){
        cin >> c;
        cs.push_back(c);
    }
    ll bestans = 1e14;
    ll ans;
    for(ll i = 0; i <= k; i++){
        ans = 0;
        for(ll j = i; j < n; j+=(k*2 + 1)){
            ans+=cs[j];
        }
        bestans = min(bestans, ans);
    }
    cout << bestans << endl;
    return 0;
}