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

public class MainClass
{
	public const long Giri = 1000000007;
	public const long DpInf = 99999999999;
	
	public static void Main(string[] args)
	{
		var n = Console.ReadLine().ToInt();
		var a = Input().ToInt();
		
		int cnt = 0;
		for(int i = 0; i < n - 1; i++)
		{
			if(i+1 == a[i])
			{
				var k = a[i+1];
				a[i+1] = a[i];
				a[i] = k;
				cnt++;
			}
		}
		Console.WriteLine(cnt);
	}
	
	static long GCD(long a, long b)
	{
		var v = new[] { a, b };
		while (v[1] != 0) { v = new[] { v[1], v[0] % v[1] }; }
		return v[0];
	}
	public static string[] Input() { return Console.ReadLine().Split(); }
}

public static class Smarter
{
	public static int ToInt(this string str) { return int.Parse(str); }
	public static long ToLong(this string str) { return long.Parse(str); }
	public static int[] ToInt(this string[] str)
	{
		var r = new int[str.Length];
		for(int i = 0; i < str.Length; i++) r[i] = str[i].ToInt();
		return r;
	 }
}
