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

long long c[10000];
int main() {
    int n,k;
    cin >> n >> k;
    for(int i = 0; i < n; i++){
        cin >> c[i];
    }
    long long cost = 0;
    for(int i = 0; i < n;){
        if(c[i]>c[i+k]){
            cost += c[i+k];
            i = i+2*k+1;
        }
        else{
            cost += c[i];
            i = i+k+1;
        }
            
    }
    cout << cost << endl;
    return 0;
}