#include<iostream>
using namespace std;
int main()
{
    int n,k,en;
    cin>>n>>k;
    en=n-1;
    long long int c[n],bcon[n],s[n],bs[n],myse,cost=0;
    int setp,pos=0,spos,epos;
    float cpb[n];
    for(int i=0;i<=en;i++)
    {bcon[i]=1;
     bs[i]=i;
     s[i]=1;
        cin>>c[i];
     if(i>=k)
         bcon[i]+=k;
     else
         {
         bcon[i]+=i;
     }
     if(i<=en-k)
         bcon[i]+=k;
     else
         bcon[i]+=(en-i);
     cpb[i]=(1.0*c[i])/bcon[i];
     //cout<<cpb[i]<<endl;
     //cout<<"hello";
    }
    int calc;
    repeat:
        {
    //cout<<"hello";
    calc=0;
    for(int i=0;i<n;i++){
            if(s[i]==-1){
                calc++;
                continue;
            }
        int j=i;
        while(j>=i-k && j>=0)
            {
            if(cpb[bs[j]]>cpb[i])
                bs[j]=i;
            j--;
        }
        //cout<<"hello";
        j=i;
        while(j<=i+k && j<n){
            if(cpb[bs[j]]>cpb[i])
                bs[j]=i;
            j++;
        }
        //cout<<"hello";
    }
    //cout<<"hello";
    if(calc==n)
    {
        cout<<cost;
        return 0;
    }
    setp=0;
    for(int i=0;i<n;i++){
     if((s[i]!=-1)&&((setp==0)||(cpb[bs[i]]>cpb[pos])))
         {
         setp=1;
         pos=bs[i];
         //cout<<pos<<"pos"<<endl;
     }
    }
    //cout<<pos;
    myse=pos;
    cost+=c[pos];
    s[pos]=0;
    while(myse>=pos-k && myse>=0)
    {
        s[myse]=0;
        myse--;
    }
    myse=pos;
    while(myse<=pos+k && myse<n)
        {s[myse]=0;myse++;}
    spos=pos-k;
    if(spos<0)
        spos=0;
    epos=pos+k;
    if(epos>=n)
        epos=n-1;
    for(int i=spos;i<=epos;i++)
    {
        bcon[i]=0;
        int starter,ender;
        starter=i-k;
        if(starter<0)
            starter=0;
        ender=i+k;
        if(ender>=n)
            ender=n-1;
        for(int j=starter;j<=ender;j++){
            if(s[j]==1)
                bcon[i]++;
        }
        if(bcon[i]==0)
            s[i]=-1;
        else
        {
            cpb[i]=(1.0*c[i])/bcon[i];
        }
    }
        }
        goto repeat;
}