using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using static System.Math;
using static System.Console;
using static CS_Contest.Utils;
using System.IO;
//using static CS_Contest.Library;

namespace CS_Contest {
	using Li = List<int>;
	using LLi = List<List<int>>;
	using Ls = List<string>;
	using Ll = List<long>;
	using LLl = List<List<long>>;
	class Program {
		static void Main(string[] args) {
			var sw = new StreamWriter(OpenStandardOutput()) { AutoFlush = false };
			SetOut(sw);
			new Calc().Solve();
			Out.Flush();
			return;
        }

		private class Calc {
			public void Solve() {
				int N = ReadInt();
				var list = ReadInt(' ').ToIndexEnumerable().ToList();

				var WAlist = list.Where(x => x.Index + 1 == x.Value).ToList();

				int count = 0;

				//隣り合っているのを処理する
				for (int i = 0; i < WAlist.Count-1; i++) {
					if (WAlist[i].Index == WAlist[i + 1].Index-1) {
						var x = list[i];
						list[i] = new IndexT<int>(list[i + 1].Value, i);
						list[i + 1] = new IndexT<int>(x.Value, i + 1);
						i++;
						count++;
					}
				}
				WAlist = list.Where(x => x.Index + 1 == x.Value).ToList();

				while (WAlist.Count > 0) {
					foreach (var item in WAlist) {
						int idx = item.Index;
						//左隣と交換できるか？
						if (idx > 0 && list[idx - 1].Value != idx) {
							int x = list[idx].Value;
							list[idx] = new IndexT<int>(list[idx - 1].Value, idx);
							list[idx - 1] = new IndexT<int>(x, idx - 1);
							continue;
						}
						//右と交換する
						if (idx < list.Count - 1) {
							int x = list[idx].Value;
							list[idx] = new IndexT<int>(list[idx + 1].Value, idx);
							list[idx + 1] = new IndexT<int>(x, idx + 1);
						}
					}
					count += WAlist.Count;
					WAlist= list.Where(x => x.Index + 1 == x.Value).ToList();
				}

				count.WL();
				return;
			}

			
		}
	}


	public static class Utils {
		public static int ModValue = (int)(1000000007);
		public static long INF = long.MaxValue;
		public static long Mod(long x) => x % ModValue;
		public static long ModPow(long x, long n) { long tmp = 1; while (n != 0) { if (n % 2 == 1) { tmp = Mod(tmp * x); } x = Mod(x * x); n /= 2; } return tmp; }
		public static long DivMod(long x, long y) => Mod(x * ModPow(y, (long)(1e9 + 5)));
		public static void WL(this object obj) => WriteLine(obj);
		public static void WL(this string obj) => WriteLine(obj);
		public static void WL<T>(this IEnumerable<T> list) => list.ToList().ForEach(x => x.WL());
		public static int ReadInt() => int.Parse(ReadLine());
		public static List<int> ReadInt(char s) => ReadLine().Split(s).Where(x => x != "").Select(int.Parse).ToList();
		public static long ReadLong() => long.Parse(ReadLine());
		public static List<long> ReadLong(char s) => ReadLine().Split(s).Select(long.Parse).ToList();
		public static void ReadMulti(out int x, out int y) {
			var i = ReadInt(' ');
			x = i[0]; y = i[1];

		}
		public static void ReadMulti(out long x, out long y) {
			var i = ReadLong(' ');
			x = i[0]; y = i[1];
		}
		public static void ReadMulti(out int x, out int y, out int z) {
			var i = ReadInt(' ');
			x = i[0]; y = i[1]; z = i[2];
		}
		public static void ReadMulti(out int x, out int y, out int z, out int v) {
			var i = ReadInt(' ');
			x = i[0]; y = i[1]; z = i[2]; v = i[3];
		}

		public static string StringJoin<T>(this IEnumerable<T> l, string separator) => string.Join(separator, l);

		public static long GCD(long m, long n) {
			long tmp;
			if (m < n) { tmp = n; n = m; m = tmp; }
			while (m % n != 0) {
				tmp = n;
				n = m % n;
				m = tmp;
			}
			return n;
		}
		public static long LCM(long m, long n) => m * (n / GCD(m, n));

		public static void REP(int n, Action<int> act) {
			for (int i = 0; i < n; i++) {
				act(i);
			}
		}
		public static void RREP(int n, Action<int> act) {
			for (int i = n - 1; i >= 0; i--) {
				act(i);
			}
		}
		public static void REP(long n, Action<int> act) => REP((int)n, act);
		public static void RREP(long n, Action<int> act) => RREP((int)n, act);

		public static void Yes() => "Yes".WL();
		public static void No() => "No".WL();
		public static void YES() => "YES".WL();
		public static void NO() => "NO".WL();


		public static int ManhattanDistance(int x1, int y1, int x2, int y2) => Abs(x2 - x1) + Abs(y2 - y1);


		public class Vector2 {
			public int X { get; set; }
			public int Y { get; set; }
			public Vector2() { X = 0; Y = 0; }
			public Vector2(int x, int y) { X = x; Y = y; }
			public int ManhattanDistance(Vector2 v2) {
				return Utils.ManhattanDistance(X, Y, v2.X, v2.Y);
			}
			public static int ManhattanDistance(Vector2 v1, Vector2 v2) => v2.ManhattanDistance(v2);
			public double Distance(Vector2 v2) => Sqrt(Pow(X - v2.X, 2) + Pow(Y - v2.Y, 2));
			public static Vector2 Zero { get { return new Vector2(); } }
			public void Clear() { X = 0; Y = 0; }
		}

		public struct IndexT<T> {
			public T Value { get; set; }
			public int Index { get; set; }
			public IndexT(T v,int i) { Value = v; Index = i; }
		}
		public static IEnumerable<IndexT<T>> ToIndexEnumerable<T>(this IEnumerable<T> list) => list.Select((x, i) => new IndexT<T>(x, i));
		

		public static Tuple<TKey, TSource> ToTuple<TKey, TSource>(this KeyValuePair<TKey, TSource> kvp) => new Tuple<TKey, TSource>(kvp.Key, kvp.Value);
		public static IEnumerable<Tuple<TKey, TSource>> ToTupleList<TKey, TSource>(this Dictionary<TKey, TSource> d) => d.Select(_ => _.ToTuple());
	}
}
