Problem:
Score : 400 points

Problem Statement

There are N islands lining up from west to east, connected by N-1 bridges.

The i-th bridge connects the i-th island from the west and the (i+1)-th island from the west.

One day, disputes took place between some islands, and there were M requests from the inhabitants of the islands:

Request i: A dispute took place between the a_i-th island from the west and the b_i-th island from the west. Please make traveling between these islands with bridges impossible.

You decided to remove some bridges to meet all these M requests.

Find the minimum number of bridges that must be removed.

Constraints

All values in input are integers.

2 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq a_i < b_i \leq N

All pairs (a_i, b_i) are distinct.

Input

Input is given from Standard Input in the following format:

N M
a_1 b_1
a_2 b_2
:
a_M b_M

Output

Print the minimum number of bridges that must be removed.

Sample Input 1

5 2
1 4
2 5

Sample Output 1

1

The requests can be met by removing the bridge connecting the second and third islands from the west.

Sample Input 2

9 5
1 8
2 7
3 5
4 6
7 9

Sample Output 2

2

Sample Input 3

5 10
1 2
1 3
1 4
1 5
2 3
2 4
2 5
3 4
3 5
4 5

Sample Output 3

4

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AtCorder
{
	public class Program
	{
		static void Main(string[] args)
		{
			var cin = new Scanner2();
			int n = cin.Int();
			int m = cin.Int();
			List<int>[] ends = new List<int>[n];
			for (int i = 0; i < n; i++) {
				ends[i] = new List<int>();
			}

			for (int i = 0; i < m; i++) {
				int a = cin.Int() - 1;
				int b = cin.Int() - 1;
				ends[a].Add(b);
			}

			var pq = new PriorityQueue<int>(m, true);
			int ans = 0;
			for (int i = 0; i < n; i++) {
				if (pq.Count > 0) {
					if (pq.Top == i) {
						ans++;
						pq.Clear();
					}
				}

				foreach (int next in ends[i]) {
					pq.Enqueue(next);
				}
			}

			if (pq.Count > 0) {
				++ans;
			}

			Console.WriteLine(ans);
		}
	}

	public class PriorityQueue<T>
	{
		private readonly List<T> heap_;
		private readonly Comparison<T> comparison_;
		private readonly bool reverses_;

		public T Top => heap_[0];
		public int Count => heap_.Count;

		public PriorityQueue(bool reverses = false)
			: this(Comparer<T>.Default, reverses) { }

		public PriorityQueue(int capacity, bool reverses = false)
			: this(capacity, Comparer<T>.Default.Compare, reverses) { }

		public PriorityQueue(IComparer<T> comparer, bool reverses = false)
			: this(16, comparer.Compare, reverses) { }

		public PriorityQueue(Comparison<T> comparison, bool reverses = false)
			: this(16, comparison, reverses) { }

		public PriorityQueue(
			int capacity,
			Comparison<T> comparison,
			bool reverses = false)
		{
			heap_ = new List<T>(capacity);
			comparison_ = comparison;
			reverses_ = reverses;
		}

		public void Enqueue(T item)
		{
			int c = heap_.Count;
			heap_.Add(item);
			while (c > 0) {
				int p = (c - 1) >> 1;
				if (Compare(p, c) < 0) {
					Swap(p, c);
				} else {
					break;
				}

				c = p;
			}
		}

		public T Dequeue()
		{
			T ret = heap_[0];
			int n = heap_.Count - 1;

			heap_[0] = heap_[n];
			heap_.RemoveAt(n);
			for (int p = 0, c; (c = (p << 1) + 1) < n; p = c) {
				if (c != n - 1 && Compare(c + 1, c) > 0) {
					++c;
				}

				if (Compare(p, c) < 0) {
					Swap(p, c);
				} else {
					break;
				}
			}

			return ret;
		}

		public void Clear() => heap_.Clear();

		private int Compare(int x, int y)
			=> comparison_(heap_[x], heap_[y]) * (reverses_ ? -1 : 1);

		private void Swap(int x, int y)
		{
			var t = heap_[x];
			heap_[x] = heap_[y];
			heap_[y] = t;
		}
	}


	public class Scanner2
	{
		private readonly char[] delimiter_ = new char[] { ' ' };
		private readonly string filePath_;
		private string[] buf_;
		private int index_;
		Func<string> reader_;

		public Scanner2(string file = "")
		{
			if (string.IsNullOrWhiteSpace(file)) {
				reader_ = Console.ReadLine;
			} else {
				filePath_ = file;
				var fs = new StreamReader(file);
				reader_ = fs.ReadLine;
			}
			buf_ = new string[0];
			index_ = 0;
		}

		public string Next()
		{
			if (index_ < buf_.Length) {
				return buf_[index_++];
			}

			string st = reader_();
			while (st == "") {
				st = reader_();
			}

			buf_ = st.Split(delimiter_, StringSplitOptions.RemoveEmptyEntries);
			if (buf_.Length == 0) {
				return Next();
			}

			index_ = 0;
			return buf_[index_++];
		}

		public int Int() => int.Parse(Next());
		public long Long() => long.Parse(Next());
		public double Double() => double.Parse(Next());

		public int[] ArrayInt(int N, int add = 0)
		{
			int[] Array = new int[N];
			for (int i = 0; i < N; i++) {
				Array[i] = Int() + add;
			}
			return Array;
		}

		public long[] ArrayLong(int N, long add = 0)
		{
			long[] Array = new long[N];
			for (int i = 0; i < N; i++) {
				Array[i] = Long() + add;
			}
			return Array;
		}

		public double[] ArrayDouble(int N, double add = 0)
		{
			double[] Array = new double[N];
			for (int i = 0; i < N; i++) {
				Array[i] = Double() + add;
			}
			return Array;
		}

		public void Save(string text)
		{
			if (string.IsNullOrWhiteSpace(filePath_)) {
				return;
			}

			File.WriteAllText(filePath_ + "_output.txt", text);
		}
	}
}