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

#define INF (1LL << 60)

long long cost[10005];
int N,k;

 bool TotalCost(int start,long long &res) {
    int reach = start + k;
    int leftOver = 0;
    res = 0;
    res += cost[start];
    while(reach < N-1) {
        leftOver = (N - 1) - reach;
        if( leftOver <= k) {
            return false;
        } else {
            if(leftOver > 2 * k + 1) {
                reach += 2 * k + 1;
                res += cost[reach - k];
            } else {
                reach += k+1;
                res += cost[reach];
                break;
            }
        }
    }
     return true;
}

long long solve() {
    int upperBound = min(k,N-1);
    long long minCost = INF;
    long long res = 0;
    for(int i = 0; i <= upperBound; ++i) {
        if(TotalCost(i,res)) {
            minCost = min(minCost,res);
        }

    }
    return minCost;
}

int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    cin >> N >> k;
    for(int i = 0; i < N; i++) {
        cin >> cost[i];
    }
    cout << solve() << endl;
    return 0;
}