#include <fstream>
#include <deque>

using namespace std;

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

int n, k, x;
long long sum = 0;

deque<pair<int, int>> d;

void pune_in_deque(int i, int x) {
  while (!d.empty() && d.back().second > x) {
    d.pop_back();
  }
  d.push_back(make_pair(i, x));
}

void sterge_din_deque(int position) {
  while (!d.empty() && d.front().first < position - k + 1) {
    d.pop_front();
  }
}


int main() {
  fin >> n >> k;

  for (int i = 1; i < k; ++i) {
    fin >> x;
    pune_in_deque(i, x);
  }

  for (int i = k; i <= n; ++i) {
    fin >> x;
    sterge_din_deque(i);
    pune_in_deque(i, x);
    sum += d.front().second;
  }

  fout << sum << "\n";
  return 0;
}