#include<bits/stdc++.h>
using namespace std;


int main() {
    
    int n,k;
    cin>>n>>k;
    long int *arr= new long int[n];
     for(int i=0;i<n;i++)
     {
         cin>>arr[i];
     }
    long int cost=0;
       for(int i=0;i<n;)
       {
           int  t=2*k;
           cost=cost+  *min_element(arr+i,arr+t+1);
            // cout<<*min_element(arr+i,arr+t+1);
            i=i+t;
           i++;
       }/* Enter your code here. Read input from STDIN. Print output to STDOUT */   
   cout<<cost<<endl;
    return 0;
}