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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Iterator;
import java.util.Scanner;


public class Main {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		int m = sc.nextInt();
		
		Demand[] demands = new Demand[m];
		for(int i = 0; i < m; i++){
			demands[i] = new Demand(sc.nextInt(), sc.nextInt());
		}

		Arrays.sort(demands);
		int result = 1;
		//ここは絶対落とす
		int bridgeID = demands[0].end - 1;
		
		for(int i = 1; i < m; i++){
			//既に区間が破壊されていなかった場合
			if(!demands[i].hasBridge(bridgeID)){
				result++;
				bridgeID = demands[i].end - 1;
			}
		}
		
		System.out.println(result);
	}
}

class Demand implements Comparable<Demand>{
	int begin;
	int end;
	
	Demand(int a, int b){
		this.begin = a;
		this.end = b;
	}

	@Override
	public int compareTo(Demand d) {
//		return (this.end - this.begin) - (d.end - d.begin);
		return this.end - d.end;
	}
	
	public boolean hasBridge(int bid){
		return begin <= bid && end > bid;
	}
}