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

int main() {
    int n,k;
    scanf("%d %d",&n,&k);
    long long a[n],a1[n];
    for(int i=0;i<n;i++){
        scanf("%lld",&a[i]);
        a1[i] = a[i];
    }
    long long min = 0,count=0;
    while(1)
        {
        int index;
        for(int i=0;i<n;i++){
            if(a[i]<min&&a[i]!=0||min==0)
             {
                min = a[i];
                index = i;
            }
        }
        if(min!=0&&count>0) break;
        else{
        count+=a[index];
           
        for(int j = index-k;j<=index+k;j++){
            if(a[j]!=0)
            a[j] = 0;
           else
               a[j] = a1[j];
        }
        }
    }
    printf("%lld\n",count);
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */    
    return 0;
}