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

#define INF 1000000000000000LL
int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */
    int n, k;
    cin>>n>>k;
    vector<long long> cost(n);
    for(int i=0; i<n; i++)cin>>cost[i];
    
    long long mnc=INF;
    
    for(int i=0; i<=k && i<n; i++){
        
        long long cst=cost[i];
        int j=i+k+1;
        bool b=0;
        while(j<n){
            if(j+k<n){cst+=cost[j+k];j=j+2*k+1;}
            else{b=1; break;}
        }
        if(!b){
            if(cst<mnc)mnc=cst;
        }
    }
    cout<<mnc;
    return 0;
}