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 Java7 (OpenJDK 1.7.0) (judge verdict: Accepted)

import java.util.*;
 
class demand implements Comparable{
  int from;
  int to;
 
  public int compareTo(Object other){
    demand otherdemand = (demand)other;
    return to - otherdemand.to;
  }
}

public class Main {
  public static void main(String[] args) {
 	  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++){
      demand newdemand = new demand();
      newdemand.from = sc.nextInt();
      newdemand.to = sc.nextInt();
      demands[i] = newdemand;
    }

    Arrays.sort(demands);

    int ans = 0;
    int nowclear = 0;
    for(int i = 0; i < m; i++){
      int nowfrom = demands[i].from;
      int nowto = demands[i].to;
      if(nowclear < nowfrom){
        ans++;
        nowclear = nowto - 1;
      }
    }

    System.out.println(ans);
  }
}