#include <cstdio>
#include <queue>
using namespace std;

queue <long> coada;

long N, K, s, i, x;

int main() {
	freopen("deque.in","r",stdin);
	freopen("deque.out","w",stdout);
	
		scanf("%ld %ld", &N, &K);
		
		for (i = 0; i < N+1; ++i) {
			scanf("%ld", &x);
			coada.push(x);
			if (coada.size() == K) {
				s += coada.front();
				coada.pop();
			}
		}
		
		printf("%ld\n", s);
	
	return 0;
}