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

int main() {
    long long n,k,i,sum=0,pos,l,m;
    scanf("%Ld %Ld",&n,&k);
    long long a[n];
    l=2*k+1;
    for(i=0;i<n;i++)
    {scanf("%Ld",&a[i]);}
    for(i=0;i<n;i+=l)
    {
        m=l-2;pos=i+ceil(l/2.0)-1,sum+=a[pos];
    }
    printf("%Ld",sum);
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */    
    return 0;
}