#include <iostream>
#include <cassert>
#include <vector>
#include <iomanip>

using namespace std;

const int N = (int) 1e5 + 7;

typedef long long ll;

int n, k;
int cost[N];

long long dp[N];

int main() {
  #ifdef LOCAL
  freopen("in", "r", stdin);
  #endif

  scanf("%d%d", &n, &k);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &cost[i]);
    dp[i] = (ll) 1e18;
  }
  k++;
  for (int i = 1; i <= k; i++) {
    dp[i] = cost[i];
  }
  for (int i = k + 1; i <= n; i++) {
    if (i - k - k + 1 <= 0) continue;
    dp[i] = dp[i - k - k + 1] + cost[i];
  }
  ll ans = (ll) 1e18;
  for (int i = 1; i <= n; i++) {
    if (n - i + 1 <= k) {
      ans = min(ans, dp[i]);
    }
  }
  cout << ans;
  return 0;
}