#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <bits/stdc++.h>
#define ll long long int 
using namespace std;
int k,n;

ll hn(int jk,vector<ll>&arr){
   // cout<<jk<<" ";
    ll ans=arr[jk];
    jk+=k+1;
    
    while(jk<n){
        jk+=k;
        if(jk>=n)break;
        ans+=arr[jk];
        jk+=k+1;
    }
    
    return ans;
}


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */  
    cin>>n>>k;vector<ll>str,stb;
    for(int i=0;i<n;i++){
        ll cc;cin>>cc;
        str.push_back(cc);        
    }
    for(int i=n-1;i>=0;i--)stb.push_back(str[i]);
    ll ans=LLONG_MAX;
    for(int i=0;i<=min(k,n-1);i++){
        int rem=n-(i+1+k);
        if(rem<=0 || rem%(2*k+1)==0 || rem%(2*k+1)>=(k+1))ans=min(ans,hn(i,str));                
    }
    for(int i=0;i<=min(k,n-1);i++){
        int rem=n-(i+1+k);
        if(rem<=0 || rem%(2*k+1)==0 || rem%(2*k+1)>=(k+1))ans=min(ans,hn(i,stb));                
    }
    
    cout<<ans;
    return 0;
}