#include<iostream>
#include<vector>
using namespace std;



int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	
	int n, k;
	cin >> n >> k;
	
	vector<int> c(n);
	for(int i=0;i<n;i++)
		cin >> c[i];
	
	long long result = 1000000000000000000LL;
	
	for(int spos = 0;spos < n &&  spos <= k; spos++) {
		long long cres = 0;
		
		if((n-1-spos) % (2*k+1) > k)
			continue;
		
		for(int i=spos; i < n; i+=2*k+1)
			cres += c[i];
		result = min(result, cres);
	}
	
	cout << result << '\n';
	return 0;
}