#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],b[n];
    for(int i=0; i<n; i++)
    {
        scanf("%lld",&a[i]);
        b[i] = a[i];
    }
    long long min = 0,s=0;
    while(1)
    {
        int ind;
        for(int i=0; i<n; i++)
        {
            if(a[i]<min&&a[i]!=0||min==0)
            {
                min = a[i];
                ind = i;
            }
        }
        if(min!=0&&s>0) 
            break;
        else
        {
            s+=a[ind];

            for(int j = ind-k; j<=ind+k; j++)
            {
                if(a[j]!=0)
                    a[j] = 0;
                else
                    a[j] = b[j];
            }
        }
    }
    printf("%lld",s);
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */
    return 0;
}