#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N=1e4+10;
int k,n;
ll costs[N];
ll memo[N][2];
ll calc(int idx,int go)
{
    //cout<<idx<<endl;
    if(idx<0||idx>=n)return 0;
    ll &ret=memo[idx][go];
    if(ret!=-1)return ret;
    ret=costs[idx];
    if(!go)
    return ret+=calc(idx-k-1,go);
    return ret+=calc(idx+k+1,go);
}
int main() {
    cin>>n>>k;
    for(int i=0;i<n;i++)
        cin>>costs[i];
    ll mn=1e18;
    memset(memo,-1,sizeof(memo));
    for(int i=0;i<n;i++)
        mn=min(mn,calc(i-k-1,0)+costs[i]+calc(i+k+1,1));
    cout<<mn<<"\n";
    return 0;
}