#include <bits/stdc++.h>

using namespace std;

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

deque<int> Q;

int v[5000001];

int main()
{
    int n, i, k;
    long long s = 0, x;
    fin >> n >> k;
    for (i = 1; i <= n; i++)
    {
        fin >> v[i];
        Q.push_back(i);
        if (Q.front() < i - k + 1)
            Q.pop_front();
        if (i >= k)
            s += v[Q.front()];
    }
    fout << s;
}