#include <iostream>
#include <iomanip>
#include <stdio.h>
#include <set>
#include <vector>
#include <map>
#include <cmath>
#include <algorithm>
#include <memory.h>
#include <string>
#include <sstream>
#include <cstdlib>
#include <ctime>
#include <cassert>

using namespace std;

typedef long long LL;
typedef pair<int,int> PII;

#define MP make_pair
#define PB push_back
#define FF first
#define SS second

#define FORN(i, n) for (int i = 0; i <  (int)(n); i++)
#define FOR1(i, n) for (int i = 1; i <= (int)(n); i++)
#define FORD(i, n) for (int i = (int)(n) - 1; i >= 0; i--)

#define DEBUG(X) { cout << #X << " = " << (X) << endl; }
#define PR0(A,n) { cout << #A << " = "; FORN(_,n) cout << A[_] << ' '; cout << endl; }

#define MOD 1000000007
#define INF 2000000000

int GLL(LL& x) {
    return scanf("%lld", &x);
}

int GI(int& x) {
    return scanf("%d", &x);
}

using namespace std;

const int MAXN = 10010;
LL c[MAXN];

int n, k;

int main() {
    GI(n); GI(k); FOR1(i, n) GLL(c[i]);

    LL res = 1LL * INF * INF;

    FOR1(i, k+1) {
        LL curr = c[i]; int lastOn = i + k;

        while (lastOn + 2 * k + 1 <= n) {
            curr += c[lastOn + k + 1];
            lastOn += 2 * k + 1;
        }

        if (lastOn >= n) {
            res = min(res, curr);
        }
        else {
            if (lastOn + k + 1 <= n) {
                res = min(res, curr + c[lastOn + k + 1]);
            }
        }
    }

    cout << res << "\n";
    return 0;
}