#include <bits/stdc++.h>
using namespace std;

#define FOR(i, a, b) for (int i=(a); i < (b); i++)
#define RFOR(i, b, a) for (int i = (b)-1; i >= (a); i--)
#define ITER(it, a) for (__typeof(a.begin()) it = a.begin(); it != a.end(); it++)
#define FILL(a, value) memset(a, value, sizeof(a))

#define SZ(a) (int)a.size()
#define ALL(a) a.begin(), a.end()
#define MP make_pair
#define PB push_back

typedef long long LL;
typedef vector<int> VI;
typedef pair<int, int> PII;

const double PI = acos(-1.0);
const int INF = 1000 * 1000 * 1000 + 7;
const LL LINF = INF * (LL) INF;

const int MAX = 20004;

vector<PII> v[MAX];
int n,k;

LL dp[MAX];
int main()
{
	//freopen("in.txt", "r", stdin);
	//ios::sync_with_stdio(false); cin.tie(0);
	cin>>n>>k;
	FOR(i,0,n)
	{
		int c;
		cin>>c;
		int st = max(0,i-k);
		int fin = min(n-1,i+k);
		int cnt = fin-st+1;
	/*	FOR(j,st,i+1)
		{
			v[j].PB(MP(cnt,c));
	//		cerr<<j<<" "<<cnt<<" "<<c<<endl;
			--cnt;
		}*/
		v[st].PB(MP(cnt,c));
	//	cerr<<st<<" "<<cnt<<endl;
	}


	FOR(i,0,n+1)
	{
		dp[i]=LINF;
	}

	dp[0]=0;

	FOR(i,0,n+1)
	FOR(j,0,SZ(v[i]))
	{
		int to = i+v[i][j].first;
		LL pr = dp[i] + v[i][j].second;
	//	cerr<<" -> "<<i<<" "<<to<<" "<<pr<<endl;
		dp[to] = min(dp[to],pr);
	}

//	FOR(i,0,n+1)cerr<<dp[i]<<" ";

	cout<<dp[n];
}