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.Linq;
using System.Collections.Generic;

namespace AtCoder
{
	class Programs
	{
		static void Main ()
		{
			var inp = Array.ConvertAll (Console.ReadLine ().Split (), int.Parse);
			var list = new List<Tuple<int,int>> ();
			for (int i = 0; i < inp [1]; i++) {
				var temp = Array.ConvertAll (Console.ReadLine ().Split (), int.Parse);
				list.Add (new Tuple<int,int> (temp [0], temp [1]));
			}
			list = list.OrderBy (f => f.Item2).ToList ();
			int removeBridge = list [0].Item2-1;
			int ans = 1;
			for (int i = 1; i < list.Count (); i++) {
				if (list [i].Item1 > removeBridge) {
					removeBridge = list [i].Item2 - 1;
					ans++;
				}
			}
			Console.WriteLine (ans);
		}
	}
}