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;
        for (var i = n - 1; i >= 0; --i)
        {
            if (i == 0 || a[i - 1] < a[i] - 1)
            {
                WriteLine(a[i] - 1 - i);
                return;
            }
        }
    }
}