#include <cstdio>
#include <algorithm>
#include <vector>
#include <cstdlib>
#include <cmath>

using namespace std;

typedef long long int64;

const int kMaxN = 10240;

int N, K;
int64 cost[kMaxN];
int64 dp[kMaxN];

int main() {
#ifdef LocalHost
  freopen("input.txt", "rt", stdin);
#endif

  scanf("%d %d", &N, &K);
  for (int i = 0; i < N; i++)
    scanf("%lld", &cost[i]);

  int64 res = 1e+8;
  for (int i = 0; i <= K && i < N; i++) {
    int64 c = 0;
    int last;
    for (int j = i; j < N; j += (2*K+1)) {
      last = j;
      c += cost[j];
    }

    if (N-1-last <= K)
      res = min(res, c);
  }

  printf("%lld\n", res);

  return 0;
}