#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;

int fun(int *c,int s,int e,int k)
    {
    int m=(s+e)/2;
    if(m-k<=0&&m+k>=e)
        return c[m];
    else if(m-k<=0&&m+k<e)
        return (c[m]+fun(c,m+k,e,k));
    else if(m-k>0&&m+k>=e)
        return (c[m]+fun(c,s,m-k,k));
    else
        return (c[m]+fun(c,s,m-k,k)+fun(c,m+k,e,k));
}

int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */ 
    int n,k,ans;
    cin>>n>>k;
    int c[n];
    for(int i=0;i<n;i++)
        cin>>c[i];
    cout<<fun(c,0,n-1,k);
    return 0;
}