#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 long int n,k,c;
    vector<long long int> cost;
    //map<long long long int,long long long int> hash;
    cin>>n>>k;
    for(int i=0;i<n;i++)
    {
        cin>>c;
        cost.push_back(c);
        //hash[i]=c;
    }
    long long int sum=0;
    for(int i=k;i<cost.size();i+=2*k)
    {
        sum+=cost[i];    
    }
    cout<<sum;
    return 0;
}