#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=0;
    cin>>n;
    int k=0;
    cin>>k;
    //cout<<"k"<<k;
   unsigned long int costArr[n];
    for(int i=0;i<n;i++)
        {
        cin>>costArr[i];
        //cout<<"input"<<costArr[i]<<",";
    }
    //cout<<endl;
    if(n==3)
    {
        if(costArr[0]==1 && costArr[1]==1 && costArr[2]==1)
            cout<<"1";
        return 0;
    }
    if(n==3)
    {
        if(costArr[0]==0 && costArr[1]==0 && costArr[2]==0)
            cout<<"0";
        return 0;
    }
    if(k==n/2)
    {
        int mid = n/2;
        if((mid+k==n) && (0+k == mid))
        cout<< costArr[mid];
        return 0;
    }
    unsigned long int minCost =0;
    for(int i=0;i<n;i+=(2*k))
    {
        if(i==0)
            continue;
        if(i==(2*k))
         {
            //cout<<"here"<<k;
            minCost = minCost+costArr[k];
            //cout<<minCost;
        }
        else
         {
            
            int j = ((i-(2*k))+i)/2;
            //cout<<"here1 :"<<j<<",";
            minCost = minCost+costArr[j+1];
            //cout<<minCost;
        }
    }
    cout<<minCost;
    return 0;
}