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

public class Amatsuki
{
    private static List<int> sushiList;

    public static void Main()
    {
        Console.ReadLine();
        sushiList = Console.ReadLine().Split(new char[] {' '}).Select(x => int.Parse(x)).ToList();
        var eatenSushi = 0;

        do
        {
            eatenSushi += TakeSushi();
        } while (sushiList.Sum() != 0);

        Console.WriteLine(eatenSushi);
    }

    private static int TakeSushi()
    {
        var deliciousSushi = sushiList
            .Select((x, i) => new { Index = i, Sushi = x })
            .OrderByDescending(x => x.Sushi)
            .First();

        sushiList[deliciousSushi.Index] = 0;
        
        if(deliciousSushi.Index != sushiList.Count)
        {
            sushiList[deliciousSushi.Index + 1] = 0;
        }

        if(deliciousSushi.Index != 0)
        {
            sushiList[deliciousSushi.Index - 1] = 0;
        }

        return deliciousSushi.Sushi;
    }
}