#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
int n,k;
int c[10007];
int cost;
int test(int start,int end){
    if(end>start)
        return  (c[((start+end)/2)] + test(start,((start+end)/2)-k)) ;    
    else
        return 0;
}

int main() {
    cin >> n >> k;
    for(int i=0;i<n;i++){
        cin >> c[i];
    }
    cout << test(0,n);
    return 0;
}