#include <bits/stdc++.h>
using namespace std;

#define long long long 
#define ve vector
int n, k;
ve<long> a;

int main() {
    ios_base::sync_with_stdio(false);cin.tie(0); cout.tie(0);
    cin >> n >> k;
    a.resize(n);
    for (int i = 0; i < n; ++i) cin >> a[i];
    long ans = LLONG_MAX;
    for (int i = 0; i <= k; ++i) {
        long tans = 0;
        for (int f = i; f < n; f += 2 * k + 1) tans += a[f];
        ans = min(ans, tans);
    }
    cout << ans;
    return 0;
}