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


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