#include<bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<pii> vpii;
typedef unsigned long long llu;

#define author ayushtomar
#define rf freopen("in.txt", "r", stdin)
#define wf freopen("out.txt", "w", stdout)
#define debug(x) cerr<<#x<<" "<<x<<endl;
#define f first
#define s second
#define mp make_pair
#define pb push_back
ll C[1<<21];
int main()
{
    int n,k;
    ll ans=LONG_LONG_MAX;
    cin>>n>>k;
    for(int i=1;i<=n;i++) cin>>C[i];
    for(int i=1;i<=k+1;i++)
    {
        ll tot=0;
        int start=i;
        while(start<=n)
        {
            tot+=C[start];
            start= start + 2*k+1;
        }
        ans=min(ans,tot);
    }
    cout<<ans;

    return 0;
}