#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
const ll INF = (ll)1e18;
const int MAXN = 10 * 1000 + 7;
ll dp[MAXN];
int cost[MAXN];
vector<pair<int, int> > go[MAXN];
int main()
{
#ifdef LOCAL
	freopen("input.txt", "r", stdin);
#endif
	int n, k;
	scanf("%d %d", &n, &k);
	for (int i = 1; i <= n; i++)
	{
		scanf("%d", &cost[i]);
		int r = min(n, i + k);
		int l = max(1, i - k);
		//cerr << l << " " << r << endl;
		go[r].push_back(make_pair(cost[i], l));
		dp[i] = INF;
	}
	for (int i = 1; i <= n; i++)
	{
		for (int j = 0; j < (int)go[i].size(); j++)
		{
			cerr << i << " " << go[i][j].second << endl;
			int l = go[i][j].second;
			int w = go[i][j].first;
			for (int k = l - 1; k < l; k++)
			{
				dp[i] = min(dp[i], dp[k] + w);
			}
		}
	}
	printf("%lld\n", dp[n]);
}