#include <fstream>
#include <deque>

using namespace std;

int main (int argc, char *argv[])
{
	ifstream in("deque.in");
	ofstream out("deque.out");
	long long n, k, x, minim = 0;
	deque<long long> lista;
	
	in >> n >> k;
	for (long long i = 0; i != n; ++i)
	{
		in >> x;
		lista.push_front(x);
		if (lista.size() >= k)
		{
			minim += min(lista[0],min(lista[1],lista[2]));
			lista.pop_back();
		}
	}
	out << minim;
	return 0;	
}