#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>

int main() {

    int n,k;
    scanf("%d %d",&n,&k);
    long unsigned int arr[n];
    for(int i=0; i<n; i++){
        scanf("%lu",&arr[i]);
    }
    long unsigned int cost =0;
    int bulbs = n;
    while(bulbs){
        bulbs -= (k*2)+1;
        cost+=arr[k];
        for(int i=(k);i>0;i--) arr[i] = 0;
        for(int i=k+1; i<(k+k); i++ ) arr[i] = 0;
    }
    printf("%lu",cost);
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */    
    return 0;
}