#include <iostream>
#include <fstream>
#include <vector>
#include <stack>
#include <chrono>
#include <iomanip>
#include <unordered_map>
#include <unordered_set>
#include <set>
#include <algorithm>
#include <thread>
#include <numeric>
#include <queue>
#include <functional>
#include <cmath>

using namespace std;

#define endl "\n"
#ifndef MAIN_BEGIN
#define START_TIMER(...)
#define TIMESTAMP(...)
#define MAIN_BEGIN int main() { \
    ios::sync_with_stdio(false);
#define RETURN return 0
#define MAIN_END RETURN; }
#define OUTPUT if(false) cout
#endif

MAIN_BEGIN

    int n;
    cin >> n;
    int k;
    cin >> k;
    vector<long long> c(n);
    for(auto& i : c) cin >> i;
    if(k == 0) {
        cout << accumulate(c.begin(), c.end(), 0LL);
        RETURN;
    }
    long long best = 999999999999LL;
    for(int a=0; a<=k; a++) {
        if(a >= n) break;
        long long cost = c[a];
        long long current = a;
        long long lastCovered = a+k;
        bool did = true;
        while(lastCovered < n-1) {
            lastCovered += 2*k+1;
            current += 2*k+1;
            if(current >= n) {
                did = false;
                break;
            }
            cost += c[current];
        }
        if(did) {
            if(cost < best) best = cost;
        }
    }
    cout << best;

MAIN_END