#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,i,c=0,a[10000];
    cin>>n>>k;
    for(i=0; i<n; i++)
        {
        cin>>a[i];
    }
    for(i=k; i<n; i=i+k+1)
     {
        c=c+a[i];
    }   
    cout<<c<<endl;
    return 0;
}