#include<iostream>
#include<algorithm>

using namespace std;

int dp[1010] = { 0 };
int V[1010] = { 0 };
int N;

int max(int k) {
	if (k >= N) return 0;
	if (dp[k] != 0)return dp[k];
	if (k == N-1) {
		dp[k] = V[N-1];
		return V[N-1];
	}
	int ans =  max(V[k] + max(k + 2), V[k + 1] + max(k + 3));
	dp[k] = ans;
	return ans;
}

int main() {
	cin >> N;
	for (int i = 0; i < N; i++)
	{
		cin >> V[i];
	}
	cout << max(0) << endl;
	return 0;
}