#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> v(n+1);
    for(int i=1 ;i<=n ;i++)
        cin>> v[i];
    int ans=0 ;
    int i=0 ;
    while(2*k+i+1<=n){ 
        ans+=v[i+k+1];
        i += (2*k+1);
    }
    cout<<ans<<endl;
    return 0;
}