#include <fstream>
#include <deque>

using namespace std;

deque<pair<int,int> > q;

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

int main()
{
    long long s=0;
    int n,x,i,k;
    cin>>n>>k;
    for(i=1;i<=n;i++)
    {
        cin>>x;
        while(!q.empty() && q.back().second)
            q.pop_back();
        q.push_back({i,x});
        if(q.front().first == i-k)
            q.pop_front();
        if(i>=k)
            s+=q.front().second;
    }
    cout<<s;
    return 0;
}