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


int main() {
    long long ans=10000000000000000,n,k,c[10004],i,j,dp[10004];
    cin>>n>>k;
    for(i=0;i<n;i++){
        cin>>c[i];
    dp[i]=10000000000000;}
    for(i=0;i<k+1;i++){
        long long p=0;
        for(j=i;j<n;j+=2*k+1){
        p+=c[j];
    }
        ans=min(p,ans);
    }
cout<<ans<<endl;
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    return 0;
}