#include <bits/stdc++.h>
#define pb(x) push_back(x)
#define bug cout<<"HERE"<<endl;
#define SSTR( x ) static_cast< std::ostringstream & >( \
        ( std::ostringstream() << std::dec << x ) ).str()
#define fast 	ios_base::sync_with_stdio(0); cin.tie(0);
#define FOR(i,n) for(int i=0;i<n;++i)
#define FOR2(i,x,n) for(int i=x;i<n;++i)
#define FOR3(i,n) for(int i=n;i>=0;--i)
#define inp freopen("input.txt", "r", stdin);
#define out freopen("output.txt", "w", stdout);
#define PI = acos(-1.0);
using namespace std;

typedef long long ll;
typedef vector<int> vi;


int n , k ;
const int MAX = 10001;
ll arr[MAX];

int main() {

	scanf ("%d %d" , &n , &k);
	FOR (i , n) scanf ("%d" , &arr[i]);
	ll mn = 1e17 ;
	FOR (i , min (k + 1 , n)) {
		ll here = 0 ;
		bool covered = 0 ;
		for (int j = i ; j < n ; j += (k * 2) + 1) {
			if (j + k >= n - 1) covered = 1 ;
			here += arr[j];
		}
		if (covered) mn = min (mn , here);
	}
	printf ("%lld\n" , mn);
	return 0;
}