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


int main() {
    long n,k,i;
    cin>>n>>k;
    long c[n];
    long cost = 0,ans = 0;
    for(i=0;i<n;i++){
        cin>>c[i];
        cost += c[i];
    }
    if(k==0)
        cout<<cost;
    else{
        if(c[n/2]<c[n/2+1])
            i = n/2;
        else
            i = n/2+1;
        while(i<n){
            ans += c[i];
            i += k;
        }
        cout<<ans;
    }
    return 0;
}