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: Wrong Answer)

import java.util.*;

class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int islands = sc.nextInt();
    int counter = sc.nextInt();
    ArrayList<Bridge> list = new ArrayList<>();
    list.add(new Bridge(sc.nextInt(),sc.nextInt()));
    
    for(int i = 0;i<counter-1;i++){
      int front = sc.nextInt();
      int rear = sc.nextInt();
      boolean make = true;
      for(int k = 0;k<list.size();k++){
        if(list.get(k).getFront()>= rear
           ||list.get(k).getRear() <= front){
          continue;
        }else{
          make = false;
          if(front > list.get(k).getFront()){
            list.get(k).setFront(front);
          }
          if(rear < list.get(k).getRear()){
            list.get(k).setRear(rear);
          }
          break;
        }
      }
      if(make) list.add(new Bridge(front,rear));
    }
    System.out.println(list.size());
  }
}

class Bridge{
  private int front;
  private int rear;
  
  public Bridge(int font,int rear){
    this.front = front;
    this.rear = rear;
  }
  
  public void setFront(int front){
    this.front = front;
  }
  
  public void setRear(int rear){
    this.rear = rear;
  }
  
  public int getFront(){
    return this.front;
  }
  
  public int getRear(){
    return this.rear;
  }
}