#include <iostream>
#include <fstream>
#include <deque>

using namespace std;

ifstream in("deque.in");
ofstream out("deque.out");

int main() {
	int n, subsir;
	long long s = 0;
	in >> n >> subsir;
	deque<int> d(n);
	for (int i{}; i < n; ++i) {
		in >> d[i];
	}
	while (d.size() >= subsir){
		int x, y, z;
		x = *(d.begin());
		y = *(d.begin() + 1);
		z = *(d.begin() + 2);
		s += min(min(x, y), z);
		d.pop_front();
	}
	out << s;
}