#include <bits/stdc++.h>
#define ll long long
#define maxn 100005
#define slld(t) scanf("%lld",&t)
#define sd(t) scanf("%d",&t)
#define pd(t) printf("%d\n",t)
#define plld(t) printf("%lld\n",t)
#define fo(i,n) for(long long i=0;i<n;i++)
using namespace std;
ll dp[100000];
ll arr[100000];
void solve(){
    ll infy=1000000000000;
    ll n,k;
    slld(n);
    slld(k);
    fo(i,n){
        slld(arr[i]);
    }
    ll an=infy;
    if(n<=k+1){

        for(int i=0;i<n;i++)an=min(an,arr[i]);
        plld(an);
        return ;
    }

    for(int i=0;i<=k;i++){
        ll temp=arr[i];
        ll t=n-1-i-k;
        while(true){
            if(t==0){
                an=min(an,temp);
                break;
            }
            if(t<=k){
                temp=infy;
                break;
            }
            if(t>=k+k+1){
                i+=2*k + 1;
                temp+=arr[i];
                t-=2*k + 1;
                continue;
            }
            i+=2*k + 1;
            temp+=arr[i];
            an=min(an,temp);
            break;

        }
    }
    plld(an);

}
int main(){
    int t;
    t=1;
    while(t--)solve();
}