#pragma warning disable

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.Serialization.Formatters.Binary;
using System.Text;
using System.Text.RegularExpressions;
using System.Diagnostics;
using System.Numerics;
using System.Collections;

static class MainClass
{
    public static void Main(string[] args)
    {


        var n = Console.ReadLine().ToInt32();
        var pn = Console.ReadLine().SplittedStringToInt32List();
        var count = 0;
        var isok = false;
        for (int i = 0; i < n; i++)
        {
            if (pn[i] - 1 == i)
                break;
            if (i == n - 1)
            {
                Console.WriteLine(0);
                return;
            }
        }
        while (!isok)
        {
            isok = true;
            for (int i = 0; i < n; i++)
            {
                if (pn[i] == i + 1)
                {
                    isok = false;
                    count++;
                    if (i == n - 1)
                    {
                        var uuu = pn[i];
                        var eee = pn[i - 1];
                        pn[i] = eee;
                        pn[i - 1] = uuu;
                    }
                    else if (i == 0)
                    {
                        var uuu = pn[i];
                        var eee = pn[i + 1];
                        pn[i + 1] = uuu;
                        pn[i] = eee;
                    }
                    else
                    {
                        var uuu = pn[i];
                        var eee = pn[i + 1];
                        var iii = pn[i - 1];
                        if (iii == i - 1 + 1)
                        {
                            pn[i] = iii;
                            pn[i - 1] = uuu;
                        }
                        else if (eee == i + 1 + 1)
                        {
                            pn[i] = eee;
                            pn[i + 1] = uuu;
                        }
                        else
                        {
                            pn[i] = iii;
                            pn[i - 1] = uuu;
                        }
                    }
                }
            }
        }
        Console.WriteLine(count);

        Console.ReadLine();

    }
    

    #region ライブラリ
    public static long ToInt64(this string str) => long.Parse(str);
    public static int ToInt32(this string str) => int.Parse(str);
    public static BigInteger ToBigInteger(this string str) => BigInteger.Parse(str);
    public static List<string> SplittedStringToList(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).ToList();
    public static List<int> SplittedStringToInt32List(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => int.Parse(x)).ToList();
    public static List<long> SplittedStringToInt64List(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => long.Parse(x)).ToList();
    public static List<BigInteger> SplittedStringToBigInteger(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => BigInteger.Parse(x)).ToList();
    public const int INF = int.MaxValue / 2;
    public const long LONGINF = long.MaxValue / 2;
    
  
  

    public static void PrintArray(int[,] array)
    {
        for (int i = 0; i < array.GetLength(0); i++)
        {
            var sb = new StringBuilder();
            for (int j = 0; j < array.GetLength(1); j++)
            {
                sb.Append(array[i, j]).Append(" ");
            }
            Console.WriteLine(sb.ToString());
        }
    }

    #endregion
}



Measured memory_bytes: 17632