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.Generic;
using System.Linq;

namespace abc103_d
{
	class Program
	{
		static void Main(string[] args)
		{
			int[] x = Console.ReadLine().Split().Select(int.Parse).ToArray();
			int[] first = new int[x[1]]; int[] second = new int[x[1]];
			for (int i = 0; i < x[1]; i++)
			{
				int[] a = Console.ReadLine().Split().Select(int.Parse).ToArray();
				first[i] = a[0]; second[i] = a[1] - 1;
			}
			int ret = 0;
			Array.Sort(second, first);
			while (first.Length != 0)
			{
				ret++;
				List<int> a = first.ToList(); List<int> b = second.ToList();
				int time = b[0];
				for (int i = 0; i < a.Count; i++)
				{
					if (a[i] <= time)
					{
						a.RemoveAt(i); b.RemoveAt(i);
						i--;
					}
				}
				first = a.ToArray(); second = b.ToArray();
			}
			Console.WriteLine(ret);
		}
	}
}