#include <iostream>
#include <fstream>
#include <math.h>
using namespace std;

int main() {
	ifstream in("input.txt");
	ofstream out("output.txt");
	int n = 0;
	in >> n;
	if (n > 0) {
		int *array = new int[n];
		int *res_array = new int[n];
		for (int i = 0; i < n; i++) {
			res_array[i] = -1;
			in>>array[i];
		}
		res_array[0] = array[0];
		res_array[2] = array[2] + array[0];
		for (int i = 3; i < n; i++) {
			res_array[i] = fmax(res_array[i - 3], res_array[i - 2]) + array[i];
		}
		out << res_array[n - 1];
	}
	else {
		out << -1;
	}
	system("pause");
}