#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 */   
    int n,k;
    cin>>n>>k;
    long long int c[n];
    for(int i=0;i<n;i++){
        cin>>c[i];
    }
    long long int minC=1000000000;
    for(int i=0;i<=k;i++){
        int cost=c[i];
        for(int j=i+2*k+1;j<n;j+=2*k+1){
            cost+=c[j];
        }
        if(cost<minC){
            minC=cost;
        }
        
    }
    cout<<minC;
}