#include <bits/stdc++.h>
using namespace std;
#define forn(i, n) for (int i = 0; i < (int)(n); ++i)
#define fore(i, b, e) for (int i = (int)(b); i <= (int)(e); ++i)
#define ford(i, n) for (int i = (int)(n) - 1; i >= 0; --i)
#define pb push_back
#define fi first
#define se second
#define all(x) (x).begin(), (x).end()
typedef vector<int> vi;
typedef pair<int, int> pii;
typedef long long i64;
typedef unsigned long long u64;
typedef long double ld;
typedef long long ll;
// #define sz(x) int((x).size())

const int maxn = 100500;

int n;

i64 rmq[maxn*2];
i64 get(int l, int r) {
    i64 res = 1e18;
    l += n, r += n;
    while (l < r) {
        if (l%2 == 1) res = min(res, rmq[l]);
        if (r%2 == 0) res = min(res, rmq[r]);
        l = (l+1)/2;
        r = (r-1)/2;
    }
    if (l == r) res = min(res, rmq[l]);
    return res;
}
void put(int i, i64 x) {
    i += n;
    rmq[i] = min(rmq[i], x);
    for (i /= 2; i; i /= 2) rmq[i] = min(rmq[i*2], rmq[i*2+1]);
}

int k;
int c[maxn];
i64 d[maxn];

vector<pii> act[maxn]; // cost, l

i64 solved() {
    i64 mn = 1e18;
    forn(mask, 1<<n) {
        i64 s = 0;
        forn(i, n) if (mask&(1<<i)) s += c[i];
        if (mn < s) continue;
        bool ok = true;
        forn(i, n) {
            int cnt = 0;
            forn(j, n) if ((mask&(1<<j)) && abs(i-j) <= k) {
                ++cnt;
            }
            if (cnt != 1) {
                ok = false;
                break;
            }
        }
        if (ok) mn = s;
    }
    return mn;
}

int main() {
#ifdef LOCAL
    freopen("d.in", "r", stdin);
#endif

    cin >> n >> k;
    forn(i, n) cin >> c[i];
    i64 res = 1e18;
    forn(st, min(k+1, n)) {
        i64 s = 0;
        bool ok = true;
        for (int i = st; ; i += 2*k+1) {
            s += c[i];
            if (abs((n-1) - i) <= k) {
                ok = i < n;
                break;
            }
        }
        if (ok) res = min(res, s);
    }
    cout << res << endl;
//     int t =solved();
//     cerr << t << endl;
//     assert(res == t);

#ifdef LOCAL
    cerr << "Time elapsed: " << clock() / 1000 << " ms" << endl;
#endif
    return 0;
}