import java.util.*;
import java.lang.*;
import java.io.*;

class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		int n;
		Scanner in = new Scanner(System.in);
		n = in.nextInt();
		int[] dp = new int[n + 1];
		for(int i = 1; i <= n; ++i){
			dp[i] = in.nextInt();
		}
		dp[1] = -1;
		for(int i = 3; i<=n; ++i){
			dp[i] = dp[i] + Math.max(dp[i-2], dp[i-3]);
		}
		System.out.println(dp[n]);
	}
}