#include <fstream>
#include <cstring>
#include <algorithm>
#include <deque>
using namespace std;

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

int n, m;
int sol;
deque<pair<int, int>> s;

int main() {

    fin >> n >> m;
    for (int i = 1; i < m; ++i) {
        int k;
        fin >> k;
        while (!s.empty() && k < s.back().first) {
            s.pop_back();
        }
        s.push_back({k, i});
    }

    for (int i = m; i <= n; ++i) {
        int k;
        fin >> k;
        while (!s.empty() && k < s.back().first) {
            s.pop_back();
        }
        s.push_back({k, i});
        while (s.front().second <= i - m) {
            s.pop_front();
        }
        sol += s.front().first;
    }
    fout << sol;

    fout.close();
    return 0;
}