#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#define ll long long int
using namespace std;


int main() {
    int n,k;
    ll ans = 1000000001;
    cin >> n >> k;
    vector<ll> A(n);
    for(int i=0;i<n;i++) {
        cin >> A[i];
        ans = min(ans,A[i]);
    }
    if(n <= k) cout << ans << endl;
    else{
        ans = 0;
        for(int i=k;i<n;i+=2*k){
            ans += A[i];
        }
        cout << ans << endl;
    }
    return 0;
}