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

int main() {
	int n; cin >> n;  int b; cin >> b;
	vector<int> arr(n);
	for(int i = 0; i < n; i++)
	cin >> arr[i];
	
	int sum = 0; 
	for(int i = 0; i < b; i++)
	sum += arr[i];
	
	int maxs = sum;
	for(int i = 1; i <= b; i++){
		sum = sum - arr[b-i] + arr[n-i];
		maxs = max(maxs, sum);
	}
	cout << maxs;
	return 0;
}