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

int main() {

    /* Enter your code here. Read input from STDIN. Print output to STDOUT */  
    int n,k;
    scanf("%d %d",&n,&k);
    long long a[n],count=0;
    for(int i=0;i<n;i++)
        scanf("%lld",&a[i]);
    for(int i=0;i<n;i+=k*2+1){
        count+=a[i+k];
    }
    printf("%lld",count);
    return 0;
}