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


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */ 
    long long int a[100001]={0},b[100001],k,i=0,n,ans=1000000000000000,j;
    cin>>n>>k;
    for(i=0;i<n;i++)
        cin>>b[i];
    for(i=0;i<2*k+1;i++)
        a[i]+=b[i];
    for(i=0;i<=n-2-2*k;i++)
    {
        a[i+2*k+1]+=b[i+2*k+1]+a[i];
    }
    //if(n==)
    //for(;i<n;i++)
      //  a[i]+=b[i];
    for(i=n-1;i>=0&&i>=n-k-1;i--)
    {
        if(ans>a[i])
        {
            ans=a[i];
            j=i;
        }
    }
    //for(i=0;i<n;i++)
      //  cout<<a[i]<<" ";
    cout<<ans;
    return 0;
}