using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var a = NList;
        if (n == 1)
        {
            WriteLine(0);
            return;
        }
        var count = new int[n / 2];
        foreach (var ai in a) if (ai < count.Length) ++count[ai];
        var tmp = 0;
        for (var i = 0; i < count.Length; ++i)
        {
            tmp += Math.Max(0, 2 - count[i]);
            if (tmp >= 2)
            {
                WriteLine(i);
                return;
            }
        }
        WriteLine(count.Length);
    }
}