#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    int n, k;
    cin>>n>>k;
    vector<int> c(n);
    for (auto i = 0; i<n; i++) cin>>c[i];
    int tot = 0;
    for (auto i = k; i<n; i += 2*k+1) {
        tot += c[i];
    }
    cout<<tot;
    return 0;
}