#include <iostream>
#include<bits/stdc++.h>
using namespace std;
 
int main() {
	// your code goes here
	int n,b;
	cin>>n>>b;
	vector<int> prefix(n+1,0);
	vector<int> suffix(n+1,0);
 
	vector<int> nums(n);
	for(int i=0;i<n;i++){
		cin>>nums[i];
	}
 
	for(int i=1;i<=n;i++){
		prefix[i]=prefix[i-1]+nums[i-1];
	}
 
	for(int i=n-1;i>=0;i--){
		suffix[i]=suffix[i+1]+nums[i];
 
	}
 
	int i=b;
	int j=n;
	int ans = INT_MIN;
 
 
	while(i>=0){
		ans = max(ans,prefix[i]+suffix[j]);
		i--;
		j--;
	}
	cout<<ans<<endl;
 
	return 0;
}

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NSAzIAo1IC0yIDMgMSAy

5 3 
5 -2 3 1 2