#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 */
    long int n,k,i,ans=0;
    cin>>n>>k;
    long int c[n];
    for(i=1;i<=n;i++)
    {
        cin>>c[i];
    }
    i=k+1;
    for(;i<=n;i+=(2*k)+1)
    {
        ans+=c[i];

    }
    cout<<ans;
    return 0;
}