#include <iostream>
#include <deque>
#include <fstream>
using namespace std;
typedef long long int big;

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

deque < int > DQ;
int v[5000005], n, k;
big S;

int main()
{
    f >> n >> k;
    for(int i = 1; i <= n; ++i)
        f >> v[i];
        DQ.push_back(1);
    for(int i = 2; i <= n; ++i)
    {
        while(i <= DQ.back())
            DQ.pop_back();
        DQ.push_back(i);
        if(DQ.front() == i - k)
            DQ.pop_front();
        if(i >= k)
            S += v[DQ.front()];
    }
    g << S;
}