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.Arrays;
import java.util.Scanner;

public class Main {
	
	static class Islands implements Comparable<Islands>{
		int a, b;
		public Islands(int a, int b) {
			this.a=a;
			this.b=b;
		}
		
		public int compareTo(Islands s) {
			return (this.b==s.b)? this.a-s.a:this.b-s.b;
		}
	}
	
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int N=sc.nextInt();
		int M=sc.nextInt();
		Islands[] land=new Islands[M];
		for(int i=0; i<M; i++) {
			int a=sc.nextInt();
			int b=sc.nextInt();
			land[i]=new Islands(a, b);
		}
		Arrays.sort(land);
		int last=0, count=0;
		
		for(int i=0; i<M; i++) {
			if(land[i].a>last) {
				last=land[i].b-1;
				count++;
			}
		}
		
		System.out.println(count);
		
		
		sc.close();
	}	
}