#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, ak, ans=0;
    cin>>n>>k;
    vector<int> v;
    for(int i=0;i<n;i++){
        cin>>ak;
        v.push_back(ak);
    }
    for(int i=k;i<n;i+=2*k+1){
        ans+=v[i];
    }
    cout<<ans;
    return 0;
}