#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    int n, k;
    cin >> n >> k;
    
    vector <long long> c(n);
    for (int i = 0; i < n; ++i) {
        cin >> c[i];
    }
    
    if (2 * k + 1 >= n) {
        long long ans = 1e18;
        for (int i = 0; i < n; ++i) {
            int l = max(0, i - k);
            int r = min(n - 1, i + k);
            if (l == 0 and r == n - 1) {
                ans = min(ans, c[i]);
            }
        }
        cout << ans << "\n";
    } else {
        long long ans = 0;
        for (int i = 0; i < n; ++i) {
            ans += c[i];
        }
        cout << ans << "\n";
    }
    return 0;
}