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


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */ 
    
    int n,k;
    cin>>n>>k;
    
    vector<int> vec(n);
    for(int i=0; i<n; ++i)
        cin>>vec[i];
    
    int cur = k;
    
    int cost =0;
    int prev = cur;
  while(cur < n){
      cost += vec[cur];
      prev = cur;
      cur = cur+2*k+1;
      
  }
    
    if(prev + k < n){
        int left = n - (prev+k);
        if(left >k){
            cost += vec[n-1];
        }
        else{
            cost += vec[n-left+k];
        }
    }
    
    cout<<cost<<endl;
    return 0;
}