#include <iostream>	
#include <vector>
#include <algorithm>
using namespace std;	
long long ansExm(long long n, long long k, vector <long long> a){
	vector<long long> s(n+1, 0);
	vector<long long> pref(n+1, 0);
	vector<long long> suff(n+1, 0);
	s[0]=0;
	for (int i = 1; i <= n; i++){ 
		s[i] = s[i - 1] + a[i]; 
	} 
	for(int i = k; i <= n; i++){ 
		pref[i] = max(pref[i - 1], s[i] - s[i - k]); 
	} 
	suff[n-k+1]=s[n]-s[n-k];
	for (int i = n - k + 1; i >= 1; i--) { 
		suff[i] = max(suff[i + 1], s[i + k - 1] - s[i - 1]); 
	} 
	long long best = 2e18; 
	for (int i = 1; i <= n - k + 1; i++) { 
		best = min(best, max(pref[i - 1], suff[i + k])); 
	} 
	return best;
}
int main(){
	int k, n;
	cin>>n>>k;
	vector <long long> arr(n+1, 0);
	for(int i=1; i<=n; i++){
		cin>>arr[i];
	}
	long long ans1=ansExm(n, k, arr);

	cout<<ans1;
}