#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
#define NMAX 10000

int c[NMAX];

int main() {
    int i, j, n, k, lastBulb;
    long long cmin = -1, cost;
    
    cin >> n >> k;
    for(i = 0; i < n; ++i) cin >> c[i];
    
    for(i = 0; i <= k; ++i)
    {
        lastBulb = -1;
        cost = 0;
        for(j = i; j < n; j += 2 * k + 1)
        {
            cost += 1LL * c[j];
            lastBulb = j + k;
        }
        
        if(lastBulb < n - 1) continue;
        if(cmin == -1 || cmin > cost) cmin = cost;
    }
    
    cout << cmin << '\n';
    
    return 0;
}