using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static string InputPattern = "Input5";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("4");
            WillReturn.Add("1 2 3 4");
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("4");
            WillReturn.Add("5 4 4 9");
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("7");
            WillReturn.Add("1 2 9 10 1 1 4");
        }
        else if (InputPattern == "Input4") {
            WillReturn.Add("1");
            WillReturn.Add("100");
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    struct JyoutaiDef
    {
        internal int CurrP;
        internal int SumVal;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();
        int[] VArr = InputList[1].Split(' ').Select(X => int.Parse(X)).ToArray();
        int UB = VArr.GetUpperBound(0);

        var stk = new Stack<JyoutaiDef>();
        JyoutaiDef WillPush;
        WillPush.CurrP = 0;
        WillPush.SumVal = 0;
        stk.Push(WillPush);

        int Answer = 0;

        //メモ化探索用
        var MemoDict = new Dictionary<int, int>();

        while (stk.Count > 0) {
            JyoutaiDef Popped = stk.Pop();
            if (Answer < Popped.SumVal)
                Answer = Popped.SumVal;

            for (int I = Popped.CurrP; I <= UB && I <= Popped.CurrP + 1; I++) {
                WillPush.CurrP = I + 2;
                WillPush.SumVal = Popped.SumVal + VArr[I];

                if (MemoDict.ContainsKey(WillPush.CurrP)) {
                    if (MemoDict[WillPush.CurrP] >= WillPush.SumVal)
                        continue;
                }
                MemoDict[WillPush.CurrP] = WillPush.SumVal;

                stk.Push(WillPush);
            }
        }
        Console.WriteLine(Answer);
    }
}