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

long process( vector<long>& c, long k )
{
    long n = c.size();
    
    long p = 0;
    long cost = 0;
        
    while (p+k < n)
    {
        cost += c[p+k];
        p += 2*k+1;    
    }
    
    if (p < n)
    {
        long minC = 100000000;
        
        for (int i=p; i<n; i++)
        {
            if (i-k <= p && i+k >= n )
            {
                minC = min(c[i], minC);        
            }
        }
        
        cost += minC;
    }
    
    return cost;
}

int main() 
{
    long n, k;
    cin >> n >> k;
    
    vector<long> c(n);
    vector<long> cr(n);
    
    for (int i=0; i<n; i++)
    {
        cin >> c[i];    
        cr[n-i-1] = c[i];
    }
    
    cout << min(process(c, k), process(cr, k)) << endl;
    
    return 0;
}