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

int n, k;
vector<int> c;

int main() {    
    cin >> n >> k;
    c.resize(n);
    for (int i = 0; i < n; i++){
        cin >> c[i];
    }
    LL res = numeric_limits<LL>::max();
    for (int i = 0; i < min(n, k + 1); i++){
        LL rr = 0;
        for (int j = i; j < n; j += 2 * k + 1){
            rr = rr + c[j];
        }
        res = min(res, rr);
    }
    if (res == numeric_limits<LL>::max()){
        throw 42;
    }
    cout << res << endl;
    return 0;
}