package yukicoder;

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.Scanner;

public class N45 {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int del, i, a, b;
		ArrayList<Integer> sushies = new ArrayList<Integer>();
		LinkedList<Integer> queue = new LinkedList<Integer>();
		LinkedList<Integer> visited = new LinkedList<Integer>();
		
		if(n == 1){
			System.out.println(sc.nextInt());
			return;
		}
		
		for(i = 0;i < n;++i){
			del = sc.nextInt();
			sushies.add(del);
			visited.add(0);
		}
		
		visited.set(0, sushies.get(0));
		queue.add(0);
		queue.add(1);
		
		i = 0;
		
		while(true){
			i = queue.poll();
			
			if(visited.get(i) > sushies.get(i)){
				del = visited.get(i);
			}else{
				del = sushies.get(i);
			}
			
			a = i + 2;
			b = i + 3;
			
			if(a < n && visited.get(a) < del + sushies.get(a)){
				visited.set(a, del + sushies.get(a));
				queue.offer(a);
			}
			
			if(b < n && visited.get(b) < del + sushies.get(b)){
				visited.set(b, del + sushies.get(b));
				queue.offer(b);
			}
			
			if(queue.size() < 1){
				System.out.println(Collections.max(visited));
				return;
			}
		}
	}
}