using System;
using System.Collections.Generic;
using System.Text;

public class Program
{
    public void Proc(){
        Reader.IsDebug = false;
        int susiCount = int.Parse(Reader.ReadLine());
        this.SusiList.AddRange(Reader.GetInt());
        
        int ans = this.GetAns(0);
        Console.WriteLine(ans);
    }
    
    private List<int> SusiList = new List<int>();
    
    private Dictionary<int, int> dic = new Dictionary<int, int>();
    
    private int GetAns(int idx) {
        if(dic.ContainsKey(idx)) {
            return dic[idx];
        }
        if(idx >= SusiList.Count) {
            return 0;
        }
        int ans = 0;
        
        ans = Math.Max(ans, GetAns(idx + 2) + this.SusiList[idx]);
        ans = Math.Max(ans, this.GetAns(idx+1));
        
        dic.Add(idx, ans);
        return ans;
    }

    public static void Main(string[] args)
    {
        Program prg = new Program();
        prg.Proc();
    }
}

class Reader
{
    public static bool IsDebug = true;
    
    private static System.IO.StringReader sr;
    
    public static string ReadLine() {
        if(IsDebug) {
            if(sr == null) {
                sr = new System.IO.StringReader(initStr.Trim());
            }
            return sr.ReadLine();
        } else {
            return Console.ReadLine();
        }
    }
    
    public static int[] GetInt(char delimiter = ' ') {
        string[] inpt = ReadLine().Split(delimiter);
        int[] ret = new int[inpt.Length];
        for(int i=0; i<inpt.Length; i++) {
            ret[i] = int.Parse(inpt[i]);
        }
        return ret;
    }
    
    private static string initStr = @"


7
1 2 9 10 1 1 4


    ";
}