#include<iostream>
#include<fstream>
#include<deque>
using namespace std;
int main()
{int n,k,s=0,i,v[50000000];
deque <int> my_deq;
ifstream f("deque.in");
f>>n>>k;
for(i=1;i<=n;i++)
	f>>v[i];
for(i=1;i<=n;i++)
{while(!my_deq.empty()&&v[my_deq.back()])
	my_deq.pop_back();
my_deq.push_back(i);
if (my_deq.front()==i-k) my_deq.pop_front();
if(i>=k) s=s+v[my_deq.front()];
}
ofstream g("deque.out");
g<<s;
f.close();
g.close();
return 0;
}