#include<bits/stdc++.h>
#define mp make_pair
#define PII pair<int,int>
#define fi first
#define se second
#define pb push_back
using namespace std;

//********************************************
//Error tracking
#define show(args...) { vector<string> _v = split(#args, ','); err(_v.begin(), args); }

vector<string> split(const string& s, char c) {
    vector<string> v;
    stringstream ss(s);
    string x;
    while (getline(ss, x, c))
        v.emplace_back(x);
    return move(v);
}

void err(vector<string>::iterator it) {}
template<typename T, typename... Args>
void err(vector<string>::iterator it, T a, Args... args) {
    cerr << it -> substr((*it)[0] == ' ', it -> length()) << " = " << a << '\n';
    err(++it, args...);
}
//********************************************

const int NMAX = 10005;

int n,k;
long long dp[NMAX],dp1[NMAX],sol,c[NMAX];

int main()
{
    int i,j;

   // freopen("input","r",stdin);
   // freopen("output","w",stdout);
    cin.sync_with_stdio(false);

    cin >> n >> k;
    for (i = 1; i <= n; i++) {cin >> c[i]; dp[i] = dp1[i] = 1LL << 60;}

    for (i = 1; i <= n && i <= (k + 1); i++) dp[min(n, i + k)] = min(dp[min(n,i + k)], c[i]);
    for (i = n; i >= 1 && i >= (n - k); i--) dp1[max(1, i - k)] = min(dp1[max(1,i - k)], c[i]);

    sol = min(dp1[1],dp[n]); //if there's any
    
    for (i = 1; i <= n; i++) //now full
        if ((i - 2*k - 1) >= 0)
            dp[i] = min(dp[i], c[i - k] + dp[i - 2*k  -1]);

    //for (i = 1; i <= n ; i++) cout << dp[i] << " ";
   // cout << "\n";
    sol = min(sol,dp[n]);

    //combine
    for (i = 1; i <= n; i++) sol = min(sol, dp[i] + dp1[i + 1]);

    cout << sol << "\n";
    return 0;   
}