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

int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	
	int n; cin >> n;
	vector<int> sushi(n), eaten(n);
	for(int i = 0; i < n; i++) cin >> sushi[i];
	queue<int> q;
	q.push(0);
	eaten[0] = sushi[0];
	while(!q.empty()) {
		int tmp = q.front();
		q.pop();
		int a = tmp + 2, b = tmp + 3;
		if(eaten[a] == 0 && a >= 0 && a <= n - 1) {
			q.push(a);
			eaten[a] = eaten[tmp] + sushi[a];
		}
		if(eaten[b] == 0 && b >= 0 && b <= n - 1) {
			q.push(b);
			eaten[b] = eaten[tmp] + sushi[b];
		}
	}
	sort(eaten.begin(), eaten.end());
	int ans1 = eaten.back();
	fill(eaten.begin(), eaten.end(), 0);
	q.push(1);
	eaten[1] = sushi[1];
	while(!q.empty()) {
		int tmp = q.front();
		q.pop();
		int a = tmp + 2, b = tmp + 3;
		if(eaten[a] == 0 && a >= 0 && a <= n - 1) {
			q.push(a);
			eaten[a] = eaten[tmp] + sushi[a];
		}
		if(eaten[b] == 0 && b >= 0 && b <= n - 1) {
			q.push(b);
			eaten[b] = eaten[tmp] + sushi[b];
		}
	}
	sort(eaten.begin(), eaten.end());
	int ans2 = eaten.back();
	cout << max(ans1, ans2) << endl;
	return 0;	
}